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, 1 insertions, 0 deletions

diff --git a/audio/null.h b/audio/null.h
index 5df7493a06..8570d4d293 100644
--- a/audio/null.h
+++ b/audio/null.h
@@ -33,6 +33,7 @@
 class MidiDriver_NULL : public MidiDriver_MPU401 {
 public:
 	int open() { return 0; }
+ 	bool isOpen() const { return true; }
 	void send(uint32 b) { }
 };