Fix leak that may occur when xmlnode_from_str fails

The failure may occur any time in the middle of parsing, and `xpd->current` may
not actually be pointing to the root of the parsed tree. Thus we need to walk
back up before freeing the xmlnode.

Fixes https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=34988

Testing Done:
Ran the reproducer testcase on `fuzz_xml`

Reviewed at https://reviews.imfreedom.org/r/911/