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.

2 files changed, 5 insertions, 0 deletions

diff --git a/engines/agi/sound_midi.cpp b/engines/agi/sound_midi.cpp
index 47cab0019f..050c0d6e2c 100644
--- a/engines/agi/sound_midi.cpp
+++ b/engines/agi/sound_midi.cpp
@@ -138,6 +138,10 @@ int SoundGenMIDI::open() {
 	return 0;
 }
 
+bool SoundGenMIDI::isOpen() const {
+	return _driver && _driver->isOpen();
+}
+
 void SoundGenMIDI::close() {
 	stop();
 	if (_driver)
diff --git a/engines/agi/sound_midi.h b/engines/agi/sound_midi.h
index 059d41208c..b7355a0d5d 100644
--- a/engines/agi/sound_midi.h
+++ b/engines/agi/sound_midi.h
@@ -72,6 +72,7 @@ public:
 
 	// MidiDriver interface implementation
 	int open();
+	bool isOpen() const;
 	void close();
 	void send(uint32 b);