gfileutils: Add precondition checks to g_file_test()

Otherwise g_file_test(NULL, …) causes a call to access(NULL, …) on
Linux, which is disallowed and valgrind complains about it.

https://bugzilla.gnome.org/show_bug.cgi?id=755046

glib/gfileutils.c

@@ -317,6 +317,8 @@ gboolean
 g_file_test (const gchar *filename,
              GFileTest    test)
 {
+  g_return_val_if_fail (filename != NULL, FALSE);
+
 #ifdef G_OS_WIN32
 /* stuff missing in std vc6 api */
 #  ifndef INVALID_FILE_ATTRIBUTES