Lock and unlock mutex in both wxSound threads using it

The code was completely broken as it locked the mutex in only one thread
and then tried to unlock it in another one, which made no sense, didn't
protect against anything and resulted in errors and assert failures.

Fix this by locking and unlocking the mutex in both threads before
accessing shared data or playing sound.

Closes #17990.

src/unix/sound.cpp

@@ -336,6 +336,7 @@ private:
 #if wxUSE_THREADS
 wxThread::ExitCode wxSoundAsyncPlaybackThread::Entry()
 {
+    m_adapt->m_mutexRightToPlay.Lock();
     m_adapt->m_backend->Play(m_data, m_flags & ~wxSOUND_ASYNC,
                              &m_adapt->m_status);
 
@@ -361,6 +362,7 @@ bool wxSoundSyncOnlyAdaptor::Play(wxSoundData *data, unsigned flags,
         wxThread *th = new wxSoundAsyncPlaybackThread(this, data, flags);
         th->Create();
         th->Run();
+        m_mutexRightToPlay.Unlock();
         wxLogTrace(wxT("sound"), wxT("launched async playback thread"));
         return true;
 #else