Fix leak in `purple_markup_find_tag` on error

The output is set to `NULL`, but the internal data list is not freed if it happened to get something put into it.

Fixes https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=35816

Testing Done:
Ran `pidgin_utils_fuzzer` on the test case in the bug report.

Reviewed at https://reviews.imfreedom.org/r/924/