patch 8.2.1001: Vim9: crash with nested "if" and assignment

Problem:    Vim9: crash with nested "if" and assignment.
Solution:   Skip more of the assignment.  Do not set ctx_skip when code is
            reachable.

src/testdir/test_vim9_script.vim

@@ -1162,6 +1162,26 @@ def Test_if_const_expr_fails()
   call CheckDefFailure(["if has('aaa') ? true false"], 'E109:')
 enddef
 
+def RunNested(i: number): number
+  let x: number = 0
+  if i % 2
+    if 1
+      " comment
+    else
+      " comment
+    endif
+    x += 1
+  else
+    x += 1000
+  endif
+  return x
+enddef
+
+def Test_nested_if()
+  assert_equal(1, RunNested(1))
+  assert_equal(1000, RunNested(2))
+enddef
+
 def Test_execute_cmd()
   new
   setline(1, 'default')

src/version.c

@@ -754,6 +754,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1001,
 /**/
     1000,
 /**/

src/vim9compile.c

@@ -5067,8 +5067,19 @@ compile_assignment(char_u *arg, exarg_T *eap, cmdidx_T cmdidx, cctx_T *cctx)
 
 	if (!heredoc)
 	{
-	    if (oplen > 0)
+	    if (cctx->ctx_skip == TRUE)
 	    {
+		if (oplen > 0 && var_count == 0)
+		{
+		    // skip over the "=" and the expression
+		    p = skipwhite(op + oplen);
+		    compile_expr0(&p, cctx);
+		}
+	    }
+	    else if (oplen > 0)
+	    {
+		type_T	*stacktype;
+
 		// For "var = expr" evaluate the expression.
 		if (var_count == 0)
 		{
@@ -5113,10 +5124,6 @@ compile_assignment(char_u *arg, exarg_T *eap, cmdidx_T cmdidx, cctx_T *cctx)
 			return FAIL;
 		}
 
-		if (cctx->ctx_skip != TRUE)
-		{
-		    type_T	*stacktype;
-
 		stacktype = stack->ga_len == 0 ? &t_void
 			  : ((type_T **)stack->ga_data)[stack->ga_len - 1];
 		if (lvar != NULL && (is_decl || !has_type))
@@ -5159,7 +5166,6 @@ compile_assignment(char_u *arg, exarg_T *eap, cmdidx_T cmdidx, cctx_T *cctx)
 							     cctx) == FAIL)
 		    goto theend;
 	    }
-	    }
 	    else if (cmdidx == CMD_const)
 	    {
 		emsg(_(e_const_req_value));
@@ -5220,6 +5226,10 @@ compile_assignment(char_u *arg, exarg_T *eap, cmdidx_T cmdidx, cctx_T *cctx)
 		end = p;
 	}
 
+	// no need to parse more when skipping
+	if (cctx->ctx_skip == TRUE)
+	    break;
+
 	if (oplen > 0 && *op != '=')
 	{
 	    type_T	    *expected = &t_number;
@@ -5806,7 +5816,8 @@ compile_endif(char_u *arg, cctx_T *cctx)
     }
     // Fill in the "end" label in jumps at the end of the blocks.
     compile_fill_jump_to_end(&ifscope->is_end_label, cctx);
-    cctx->ctx_skip = FALSE;
+    // TODO: this should restore the value from before the :if
+    cctx->ctx_skip = MAYBE;
 
     drop_scope(cctx);
     return arg;