patch 8.2.4163: no error for omitting function name after autoload prefix

Problem:    No error for omitting function name after autoload prefix.
Solution:   Check for missing function name. (issue #9577)

src/testdir/test_vim9_import.vim

@@ -1514,6 +1514,21 @@ def Test_vim9script_autoload_duplicate()
   delete('Xdir', 'rf')
 enddef
 
+def Test_autoload_missing_function_name()
+  mkdir('Xdir/autoload', 'p')
+
+  var lines =<< trim END
+     vim9script
+
+     def loadme#()
+     enddef
+  END
+  writefile(lines, 'Xdir/autoload/loadme.vim')
+  assert_fails('source Xdir/autoload/loadme.vim', 'E129:')
+
+  delete('Xdir', 'rf')
+enddef
+
 def Test_import_autoload_postponed()
   mkdir('Xdir/autoload', 'p')
   var save_rtp = &rtp

src/userfunc.c

@@ -3693,7 +3693,8 @@ trans_function_name(
     // Note that TFN_ flags use the same values as GLV_ flags.
     end = get_lval(start, NULL, &lv, FALSE, skip, flags | GLV_READ_ONLY,
 					      lead > 2 ? 0 : FNE_CHECK_START);
-    if (end == start)
+    if (end == start
+		   || (end != NULL && end[-1] == AUTOLOAD_CHAR && *end == '('))
     {
 	if (!skip)
 	    emsg(_(e_function_name_required));

src/version.c

@@ -750,6 +750,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    4163,
 /**/
     4162,
 /**/