libobs: Clear current async frame if cache freed

If the cache is freed the current async frame will of course become
invalid, so make sure it's set to null if the cache is freed.

libobs/obs-source.c

@@ -1627,6 +1627,7 @@ static inline void free_async_cache(struct obs_source *source)
 
 	da_resize(source->async_cache, 0);
 	da_resize(source->async_frames, 0);
+	source->cur_async_frame = NULL;
 }
 
 #define MAX_UNUSED_FRAME_DURATION 5