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, 5 insertions, 0 deletions

diff --git a/audio/mididrv.h b/audio/mididrv.h
index eed8c419f3..f635472f0e 100644
--- a/audio/mididrv.h
+++ b/audio/mididrv.h
@@ -182,6 +182,11 @@ public:
 	 */
 	virtual int open() = 0;
 
+	/**
+	 * Check whether the midi driver has already been opened.
+	 */
+	virtual bool isOpen() const = 0;
+
 	/** Close the midi driver. */
 	virtual void close() = 0;