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.

src/pulse/context.c

@@ -274,6 +274,7 @@ static void context_free(pa_context *c) {
         pa_proplist_free(c->proplist);
 
     pa_xfree(c->server);
+    pa_xfree(c->error);
     pa_xfree(c);
 }