Handle NULL intp in test_increment_intp()

Support passing NULL for &int to gtk_test_display_button_window() so
we can use that function also when we are not interested in counting
clicks.

gtk/gtktestutils.c

@@ -544,7 +544,8 @@ try_main_quit (void)
 static int
 test_increment_intp (int *intp)
 {
-  *intp += 1;
+  if (intp != NULL)
+    *intp += 1;
   return 1; /* TRUE in case we're connected to event signals */
 }