patch 8.2.1755: Vim9: crash when using invalid heredoc marker

Problem:    Vim9: crash when using invalid heredoc marker. (Dhiraj Mishra)
Solution:   Check for NULL list. (closes #7027)  Fix comment character.

src/evalvars.c

@@ -558,6 +558,7 @@ heredoc_get(exarg_T *eap, char_u *cmd, int script_get)
     int		text_indent_len = 0;
     char_u	*text_indent = NULL;
     char_u	dot[] = ".";
+    int		comment_char = in_vim9script() ? '#' : '"';
 
     if (eap->getline == NULL)
     {
@@ -585,11 +586,11 @@ heredoc_get(exarg_T *eap, char_u *cmd, int script_get)
     }
 
     // The marker is the next word.
-    if (*cmd != NUL && *cmd != '"')
+    if (*cmd != NUL && *cmd != comment_char)
     {
 	marker = skipwhite(cmd);
 	p = skiptowhite(marker);
-	if (*skipwhite(p) != NUL && *skipwhite(p) != '"')
+	if (*skipwhite(p) != NUL && *skipwhite(p) != comment_char)
 	{
 	    semsg(_(e_trailing_arg), p);
 	    return NULL;

src/testdir/test_vim9_assign.vim

@@ -732,4 +732,14 @@ def Test_assign_lambda()
   CheckScriptSuccess(lines)
 enddef
 
+def Test_heredoc()
+  var lines =<< trim END # comment
+    text
+  END
+  assert_equal(['text'], lines)
+
+  CheckDefFailure(['var lines =<< trim END X', 'END'], 'E488:')
+  CheckDefFailure(['var lines =<< trim END " comment', 'END'], 'E488:')
+enddef
+
 " vim: ts=8 sw=2 sts=2 expandtab tw=80 fdm=marker

src/version.c

@@ -750,6 +750,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1755,
 /**/
     1754,
 /**/

src/vim9compile.c

@@ -4632,6 +4632,8 @@ compile_assignment(char_u *arg, exarg_T *eap, cmdidx_T cmdidx, cctx_T *cctx)
 	eap->getline = exarg_getline;
 	eap->cookie = cctx;
 	l = heredoc_get(eap, op + 3, FALSE);
+	if (l == NULL)
+	    return NULL;
 
 	if (cctx->ctx_skip != SKIP_YES)
 	{