Fix an assert in purple_markup_html_to_xhtml

While `g_string_free` is NULL-safe, the assert for passing it in breaks
ossfuzz. This can happen if there is no `src=` in the `img` as expected.

Fixes https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=35029

Testing Done:
Ran the test case from the bug against `fuzz_html_to_xhtml`

Reviewed at https://reviews.imfreedom.org/r/921/