patch 8.2.3308: Vim9: no runtime check for argument type with varargs only

Problem:    Vim9: no runtime check for argument type if a function only has
            varargs.
Solution:   Also check argument types if uf_va_type is set. (closes #8715)

src/testdir/test_vim9_func.vim

@@ -1149,6 +1149,18 @@ def Test_call_def_varargs()
       enddef
   END
   CheckScriptFailure(lines, 'E1160:')
+
+  lines =<< trim END
+      vim9script
+      def DoIt()
+        g:Later('')
+      enddef
+      defcompile
+      def g:Later(...l:  list<number>)
+      enddef
+      DoIt()
+  END
+  CheckScriptFailure(lines, 'E1013: Argument 1: type mismatch, expected number but got string')
 enddef
 
 let s:value = ''

src/version.c

@@ -755,6 +755,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    3308,
 /**/
     3307,
 /**/

src/vim9execute.c

@@ -893,7 +893,7 @@ call_by_name(
 
     if (ufunc != NULL)
     {
-	if (ufunc->uf_arg_types != NULL)
+	if (ufunc->uf_arg_types != NULL || ufunc->uf_va_type != NULL)
 	{
 	    int i;
 	    typval_T	*argv = STACK_TV_BOT(0) - argcount;
@@ -904,7 +904,7 @@ call_by_name(
 	    {
 		type_T *type = NULL;
 
-		if (i < ufunc->uf_args.ga_len)
+		if (i < ufunc->uf_args.ga_len && ufunc->uf_arg_types != NULL)
 		    type = ufunc->uf_arg_types[i];
 		else if (ufunc->uf_va_type != NULL)
 		    type = ufunc->uf_va_type->tt_member;