patch 9.0.0742: reading past end of the line when compiling a function

Problem:    Reading past end of the line when compiling a function with
            errors.
Solution:   Do not return an invalid pointer.  Fix skipping redirection.

src/testdir/test_vim9_func.vim

@@ -4339,6 +4339,33 @@ def Test_defer()
   assert_equal('', glob('XdeferFile'))
 enddef
 
+def Test_invalid_redir()
+  var lines =<< trim END
+      def Tone()
+        if 1
+          redi =>@0
+          redi END
+        endif
+      enddef
+      defcompile
+  END
+  v9.CheckScriptFailure(lines, 'E354:')
+  delfunc g:Tone
+
+  # this was reading past the end of the line
+  lines =<< trim END
+      def Ttwo()
+        if 0
+          redi =>@0
+          redi END
+        endif
+      enddef
+      defcompile
+  END
+  v9.CheckScriptFailure(lines, 'E354:')
+  delfunc g:Ttwo
+enddef
+
 " The following messes up syntax highlight, keep near the end.
 if has('python3')
   def Test_python3_command()

src/testdir/test_vim9_script.vim

@@ -2136,15 +2136,66 @@ enddef
 
 def Test_skipped_redir()
   var lines =<< trim END
-      def T()
+      def Tredir()
         if 0
-          redir =>l[0]
+          redir => l[0]
           redir END
         endif
       enddef
       defcompile
   END
   v9.CheckScriptSuccess(lines)
+  delfunc g:Tredir
+
+  lines =<< trim END
+      def Tredir()
+        if 0
+          redir => l[0]
+        endif
+        echo 'executed'
+        if 0
+          redir END
+        endif
+      enddef
+      defcompile
+  END
+  v9.CheckScriptSuccess(lines)
+  delfunc g:Tredir
+
+  lines =<< trim END
+      def Tredir()
+        var l = ['']
+        if 1
+          redir => l[0]
+        endif
+        echo 'executed'
+        if 0
+          redir END
+        else
+          redir END
+        endif
+      enddef
+      defcompile
+  END
+  v9.CheckScriptSuccess(lines)
+  delfunc g:Tredir
+
+  lines =<< trim END
+      let doit = 1
+      def Tredir()
+        var l = ['']
+        if g:doit
+          redir => l[0]
+        endif
+        echo 'executed'
+        if g:doit
+          redir END
+        endif
+      enddef
+      defcompile
+  END
+  v9.CheckScriptSuccess(lines)
+  delfunc g:Tredir
 enddef
 
 def Test_for_loop()

src/version.c

@@ -699,6 +699,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    742,
 /**/
     741,
 /**/

src/vim9cmds.c

@@ -2411,6 +2411,8 @@ compile_redir(char_u *line, exarg_T *eap, cctx_T *cctx)
     if (lhs->lhs_name != NULL)
     {
 	if (STRNCMP(arg, "END", 3) == 0)
+	{
+	    if (cctx->ctx_skip != SKIP_YES)
 	    {
 		if (lhs->lhs_append)
 		{
@@ -2420,8 +2422,8 @@ compile_redir(char_u *line, exarg_T *eap, cctx_T *cctx)
 			return NULL;
 		}
 
-	    // Gets the redirected text and put it on the stack, then store it
-	    // in the variable.
+		// Gets the redirected text and put it on the stack, then store
+		// it in the variable.
 		generate_instr_type(cctx, ISN_REDIREND, &t_string);
 
 		if (lhs->lhs_append)
@@ -2429,8 +2431,8 @@ compile_redir(char_u *line, exarg_T *eap, cctx_T *cctx)
 
 		if (lhs->lhs_has_index)
 		{
-		// Use the info in "lhs" to store the value at the index in the
-		// list or dict.
+		    // Use the info in "lhs" to store the value at the index in
+		    // the list or dict.
 		    if (compile_assign_unlet(lhs->lhs_whole, lhs, TRUE,
 						      &t_string, cctx) == FAIL)
 			return NULL;
@@ -2440,6 +2442,7 @@ compile_redir(char_u *line, exarg_T *eap, cctx_T *cctx)
 
 		VIM_CLEAR(lhs->lhs_name);
 		VIM_CLEAR(lhs->lhs_whole);
+	    }
 	    return arg + 3;
 	}
 	emsg(_(e_cannot_nest_redir));
@@ -2465,6 +2468,12 @@ compile_redir(char_u *line, exarg_T *eap, cctx_T *cctx)
 	if (need_type(&t_string, lhs->lhs_member_type,
 					    -1, 0, cctx, FALSE, FALSE) == FAIL)
 	    return NULL;
+	if (cctx->ctx_skip == SKIP_YES)
+	{
+	    VIM_CLEAR(lhs->lhs_name);
+	}
+	else
+	{
 	    generate_instr(cctx, ISN_REDIRSTART);
 	    lhs->lhs_append = append;
 	    if (lhs->lhs_has_index)
@@ -2473,6 +2482,7 @@ compile_redir(char_u *line, exarg_T *eap, cctx_T *cctx)
 		if (lhs->lhs_whole == NULL)
 		    return NULL;
 	    }
+	}
 
 	return arg + lhs->lhs_varlen_total;
     }

src/vim9compile.c

@@ -1283,6 +1283,19 @@ vim9_declare_error(char_u *name)
     semsg(_(e_cannot_declare_a_scope_variable), scope, name);
 }
 
+/*
+ * Return TRUE if "name" is a valid register to use.
+ * Return FALSE and give an error message if not.
+ */
+    static int
+valid_dest_reg(int name)
+{
+    if ((name == '@' || valid_yank_reg(name, FALSE)) && name != '.')
+	return TRUE;
+    emsg_invreg(name);
+    return FAIL;
+}
+
 /*
  * For one assignment figure out the type of destination.  Return it in "dest".
  * When not recognized "dest" is not set.
@@ -1364,12 +1377,8 @@ get_var_dest(
     }
     else if (*name == '@')
     {
-	if (name[1] != '@'
-			&& (!valid_yank_reg(name[1], FALSE) || name[1] == '.'))
-	{
-	    emsg_invreg(name[1]);
+	if (!valid_dest_reg(name[1]))
 	    return FAIL;
-	}
 	*dest = dest_reg;
 	*type = name[1] == '#' ? &t_number_or_string : &t_string;
     }
@@ -1445,7 +1454,11 @@ compile_lhs(
     // "var_end" is the end of the variable/option/etc. name.
     lhs->lhs_dest_end = skip_var_one(var_start, FALSE);
     if (*var_start == '@')
+    {
+	if (!valid_dest_reg(var_start[1]))
+	    return FAIL;
 	var_end = var_start + 2;
+    }
     else
     {
 	// skip over the leading "&", "&l:", "&g:" and "$"