patch 9.1.0301: Vim9: heredoc start may be recognized in string

Problem:  Vim9: heredoc start may be recognized in string.
Solution: Don't skip to closing bracket for invalid list assignment.
          (zeertzjq)

closes: #14472

Signed-off-by: zeertzjq <zeertzjq@outlook.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/src/userfunc.c b/src/userfunc.c
index 015733c..71b3983 100644
--- a/src/userfunc.c
+++ b/src/userfunc.c
@@ -1230,18 +1230,15 @@
 		    int		save_sc_version = current_sctx.sc_version;
 		    int		var_count = 0;
 		    int		semicolon = 0;
-		    char_u	*argend;
 
 		    current_sctx.sc_version
 				     = vim9_function ? SCRIPT_VERSION_VIM9 : 1;
-		    argend = skip_var_list(arg, TRUE, &var_count, &semicolon,
+		    arg = skip_var_list(arg, TRUE, &var_count, &semicolon,
 									 TRUE);
-		    if (argend == NULL)
-			// Invalid list assignment: skip to closing bracket.
-			argend = find_name_end(arg, NULL, NULL, FNE_INCL_BR);
-		    arg = skipwhite(argend);
+		    if (arg != NULL)
+			arg = skipwhite(arg);
 		    current_sctx.sc_version = save_sc_version;
-		    if (arg[0] == '=' && arg[1] == '<' && arg[2] =='<')
+		    if (arg != NULL && STRNCMP(arg, "=<<", 3) == 0)
 		    {
 			p = skipwhite(arg + 3);
 			while (TRUE)