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, 1 deletions

diff --git a/audio/softsynth/adlib.cpp b/audio/softsynth/adlib.cpp
index 4a9ce54c75..9e863471f0 100644
--- a/audio/softsynth/adlib.cpp
+++ b/audio/softsynth/adlib.cpp
@@ -118,7 +118,7 @@ public:
 	byte getNumber() { return _channel; }
 	void release() { _allocated = false; }
 
-	void send (uint32 b);
+	void send(uint32 b);
 
 	// Regular messages
 	void noteOff(byte note);