Fix memory leak in context_free

Fix memory leak in context_free

In commit f727cd9a `int error` member in `struct pa_context` was changed
to `pa_context_error *error`. The memory that is allocated with
`pa_xnew0` in src/pulse/context.c:142 is never freed, and it causes
a leak of 4 bytes. The leak can be easily detected with leak-sanitizer.