patch 8.2.4009: reading one byte beyond the end of the line

Problem:    Reading one byte beyond the end of the line.
Solution:   Check for NUL byte first.

src/ex_docmd.c

@@ -3632,7 +3632,8 @@ find_ex_command(
 	}
 
 	// Check for "++nr" and "--nr".
-	if (p == eap->cmd && p[0] == p[1] && (*p == '+' || *p == '-'))
+	if (p == eap->cmd && p[0] != NUL && p[0] == p[1]
+						   && (*p == '+' || *p == '-'))
 	{
 	    eap->cmdidx = *p == '+' ? CMD_increment : CMD_decrement;
 	    return eap->cmd + 2;

src/testdir/test_vim9_func.vim

@@ -3537,6 +3537,17 @@ def Test_numbered_function_reference()
   unlet g:mydict
 enddef
 
+def Test_go_beyond_end_of_cmd()
+  # this was reading the byte after the end of the line
+  var lines =<< trim END
+    def F()
+      cal
+    enddef
+    defcompile
+  END
+  CheckScriptFailure(lines, 'E476:')
+enddef
+
 if has('python3')
   def Test_python3_heredoc()
     py3 << trim EOF

src/version.c

@@ -750,6 +750,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    4009,
 /**/
     4008,
 /**/

src/vim9compile.c

@@ -2781,7 +2781,8 @@ compile_def_function(
 	cmd = ea.cmd;
 	if ((*cmd != '$' || starts_with_colon)
 		&& (starts_with_colon || !(*cmd == '\''
-		       || (cmd[0] == cmd[1] && (*cmd == '+' || *cmd == '-')))))
+		       || (cmd[0] != NUL && cmd[0] == cmd[1]
+					    && (*cmd == '+' || *cmd == '-')))))
 	{
 	    ea.cmd = skip_range(ea.cmd, TRUE, NULL);
 	    if (ea.cmd > cmd)