Added a null check when stopping the DSPHLE plugin with the DSound back-end. Fixes the DSPHLE crash on stop.

git-svn-id: https://dolphin-emu.googlecode.com/svn/trunk@5172 8ced0084-cf51-0410-be5f-012b33b47a6e

Source/Core/AudioCommon/Src/DSoundStream.cpp

@@ -162,7 +162,8 @@ void DSound::Clear(bool mute)
 
 	if (m_muted)
 	{
-		dsBuffer->Stop();
+		if (dsBuffer != NULL)
+			dsBuffer->Stop();
 	}
 	else
 	{