Adding lock around SetCache method, which causes some crashes. Detect…

…ed in Sentry: OPENSHOT-1Q

src/Timeline.cpp

@@ -1096,6 +1096,9 @@ std::vector<Clip*> Timeline::find_intersecting_clips(int64_t requested_frame, in
 
 // Set the cache object used by this reader
 void Timeline::SetCache(CacheBase* new_cache) {
+	// Get lock (prevent getting frames while this happens)
+	const std::lock_guard<std::recursive_mutex> lock(getFrameMutex);
+
 	// Destroy previous cache (if managed by timeline)
 	if (managed_cache && final_cache) {
 		delete final_cache;