f_decoder_wrapper: fix use of destroyed mutex

After calling the main filter's destroy callback, all child filters are
destroyed. But one of them still tried to access the cache_lock mutex
(which is destroyed in said destroy callback). This actually caused a
crash on Android with _FORTIFY_SOURCE.

Fix this by destroying the child filters first.

filters/f_decoder_wrapper.c

@@ -1119,6 +1119,8 @@ static void public_f_destroy(struct mp_filter *f)
         p->dec_thread_valid = false;
     }
 
+    mp_filter_free_children(f);
+
     talloc_free(p->dec_root_filter);
     talloc_free(p->queue);
     pthread_mutex_destroy(&p->cache_lock);