Fix race conditions in NotePlayHandleManager (LMMS#4966)

NotePlayHandleManager::acquire uses a read lock unless the pool is empty.
If two threads try to acquire NotePlayHandle simultaneously
when the value of s_availableIndex is 1, one thread will try to read s_available[-1].
If the acquire action and the release action are done at the same time,
NotePlayHandleManager::acquire may try to read data
before NotePlayHandleManager::release actually writes.

This commit prevents them by always using the write lock when acquiring a NotePlayHandle.

src/core/NotePlayHandle.cpp

@@ -577,13 +577,9 @@ NotePlayHandle * NotePlayHandleManager::acquire( InstrumentTrack* instrumentTrac
 				int midiEventChannel,
 				NotePlayHandle::Origin origin )
 {
-	if( s_availableIndex < 0 )
-	{
-		s_mutex.lockForWrite();
-		if( s_availableIndex < 0 ) extend( NPH_CACHE_INCREMENT );
-		s_mutex.unlock();
-	}
-	s_mutex.lockForRead();
+	// TODO: use some lockless data structures
+	s_mutex.lockForWrite();
+	if (s_availableIndex < 0) { extend(NPH_CACHE_INCREMENT); }
 	NotePlayHandle * nph = s_available[ s_availableIndex.fetchAndAddOrdered( -1 ) ];
 	s_mutex.unlock();