patch 8.2.3974: Vim9: LISTAPPEND instruction does not check for a locked list

Problem:    Vim9: LISTAPPEND instruction does not check for a locked list.
Solution:   Check whether the list is locked. (closes #9452)

src/testdir/test_vim9_builtin.vim

@@ -78,6 +78,17 @@ enddef
 def Test_add()
   CheckDefAndScriptFailure(['add({}, 1)'], ['E1013: Argument 1: type mismatch, expected list<any> but got dict<unknown>', 'E1226: List or Blob required for argument 1'])
   CheckDefFailure(['add([1], "a")'], 'E1012: Type mismatch; expected number but got string')
+
+  var lines =<< trim END
+    vim9script
+    g:thelist = [1]
+    lockvar g:thelist
+    def TryChange()
+      g:thelist->add(2)
+    enddef
+    TryChange()
+  END
+  CheckScriptFailure(lines, 'E741:')
 enddef
 
 def Test_add_blob()

src/version.c

@@ -749,6 +749,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    3974,
 /**/
     3973,
 /**/

src/vim9execute.c

@@ -3911,12 +3911,14 @@ exec_instructions(ectx_T *ectx)
 		    list_T	*l = tv1->vval.v_list;
 
 		    // add an item to a list
+		    SOURCING_LNUM = iptr->isn_lnum;
 		    if (l == NULL)
 		    {
-			SOURCING_LNUM = iptr->isn_lnum;
 			emsg(_(e_cannot_add_to_null_list));
 			goto on_error;
 		    }
+		    if (value_check_lock(l->lv_lock, NULL, FALSE))
+			goto on_error;
 		    if (list_append_tv(l, tv2) == FAIL)
 			goto theend;
 		    clear_tv(tv2);