AUDIO: Add pure virtual MidiDriver::isOpen() method

This in turn enables modifying MidiDriver_MPU401::close() to allow
it to be called on a midi driver that has not yet been opened.

The specific issue that triggered me to make these changes was a
crash-upon-quit in HUGO, caused by it instantiating a midi driver,
then encountering an error (missing hugo.dat) *before* having
opened the new midi driver; the general cleanup code then tries
to close the (not yet opened) midi driver -> kaboom

Also fixed some engines which were leaking MidiDriver instances.

1 files changed, 4 insertions, 0 deletions

diff --git a/engines/hugo/sound.cpp b/engines/hugo/sound.cpp
index 5ef843977f..c4feb5fb1c 100644
--- a/engines/hugo/sound.cpp
+++ b/engines/hugo/sound.cpp
@@ -192,6 +192,10 @@ int MidiPlayer::open() {
 	return 0;
 }
 
+bool MidiPlayer::isOpen() const {
+	return _driver && _driver->isOpen();
+}
+
 void MidiPlayer::close() {
 	stop();
 	_mutex.lock();