gutf8: Assert that written memory stays in bounds

You may expect funny effects from passing invalid UTF-8, but not
that funny. The assert will probably be a better and more immediate
confirmation of an error than invalid writes under the address of the
string copy.

https://gitlab.gnome.org/GNOME/glib/issues/1863

glib/gutf8.c

@@ -1773,6 +1773,7 @@ g_utf8_strreverse (const gchar *str,
     {
       gchar *m, skip = g_utf8_skip[*(guchar*) p];
       r -= skip;
+      g_assert (r >= result);
       for (m = r; skip; skip--)
         *m++ = *p++;
     }