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/lure/sound.cpp b/engines/lure/sound.cpp
index 5f954eb337..482527da13 100644
--- a/engines/lure/sound.cpp
+++ b/engines/lure/sound.cpp
@@ -694,6 +694,10 @@ int MidiMusic::open() {
 	return 0;
 }
 
+bool MidiMusic::isOpen() const {
+	return _driver != 0;
+}
+
 void MidiMusic::close() {
 }