add some asserts near where some crashdumps pointed to

stepmania/src/arch/Sound/RageSoundDriver_DSound.cpp

@@ -70,6 +70,7 @@ void RageSound_DSound::Update(float delta)
 	/* SoundStopped might erase sounds out from under us, so make a copy
 	 * of the sound list. */
 	vector<stream *> str = stream_pool;
+	ASSERT(SOUNDMAN);
 	LockMutex L(SOUNDMAN->lock);
 
 	for(unsigned i = 0; i < str.size(); ++i)

stepmania/src/arch/Sound/RageSoundDriver_DSound_Software.cpp

@@ -110,6 +110,7 @@ void RageSound_DSound_Software::StartMixing(RageSound *snd)
 
 void RageSound_DSound_Software::Update(float delta)
 {
+	ASSERT(SOUNDMAN);
 	LockMut(SOUNDMAN->lock);
 
 	/* SoundStopped might erase sounds out from under us, so make a copy