output/Thread: ensure pending tags are flushed in all cases

Fixes hanging playback with soxr resampler.

Closes #139, #141

NEWS

@@ -5,6 +5,8 @@ ver 0.20.12 (not yet released)
   - vorbis: fix Tremor support
 * player
   - log message when decoder is too slow
+* output
+  - fix hanging playback with soxr resampler
 
 ver 0.20.11 (2017/10/18)
 * storage

src/output/OutputThread.cxx

@@ -271,9 +271,9 @@ try {
 inline bool
 AudioOutput::PlayChunk()
 {
-	if (tags) {
+	// ensure pending tags are flushed in all cases
 	const auto *tag = source.ReadTag();
-		if (tag != nullptr) {
+	if (tags && tag != nullptr) {
 		const ScopeUnlock unlock(mutex);
 		try {
 			ao_plugin_send_tag(this, *tag);
@@ -282,7 +282,6 @@ AudioOutput::PlayChunk()
 				    name, plugin.name);
 		}
 	}
-	}
 
 	while (command == Command::NONE) {
 		const auto data = source.PeekData();