SCUMM: Lock the sound resource while the music is playing

After some discussion on #scummvm, the player now locks the sound
resource while the music is playing. This prevents the resource
manager from expiring the resource, which at best could cause
music to restart where it shouldn't.. At worst, I guess it could
have crashed, but I never saw that happen.

engines/scumm/player_mac.cpp

@@ -25,6 +25,7 @@
 #include "engines/engine.h"
 #include "gui/message.h"
 #include "scumm/player_mac.h"
+#include "scumm/resource.h"
 #include "scumm/scumm.h"
 #include "scumm/imuse/imuse.h"
 
@@ -163,6 +164,9 @@ void Player_Mac::setMusicVolume(int vol) {
 }
 
 void Player_Mac::stopAllSounds_Internal() {
+	if (_soundPlaying != -1) {
+		_vm->_res->unlock(rtSound, _soundPlaying);
+	}
 	_soundPlaying = -1;
 	for (int i = 0; i < _numberOfChannels; i++) {
 		// The channel data is managed by the resource manager, so
@@ -194,18 +198,20 @@ void Player_Mac::startSound(int nr) {
 	Common::StackLock lock(_mutex);
 	debug(5, "Player_Mac::startSound(%d)", nr);
 
+	stopAllSounds_Internal();
+
 	const byte *ptr = _vm->getResourceAddress(rtSound, nr);
 	assert(ptr);
 
 	if (!loadMusic(ptr)) {
 		return;
 	}
 
+	_vm->_res->lock(rtSound, nr);
 	_soundPlaying = nr;
 }
 
 bool Player_Mac::Channel::loadInstrument(Common::SeekableReadStream *stream) {
-	// Load the sound
 	uint16 soundType = stream->readUint16BE();
 	if (soundType != 1) {
 		warning("Player_Mac::loadInstrument: Unsupported sound type %d", soundType);