Fixed bug 2467 - bad memcpy in SDL_OpenAudio/open_audio_device/prepare_audiospec chain

Rainer Deyke

If 'SDL_OpenAudio' is called with 'obtained == NULL', 'prepare_audiospec' performs a bad 'memcpy' with the destination and source pointing to the same block of memory.  The problem appears to be on in 'SDL_OpenAudio', which calls open_audio_device with 'obtained = desired' when 'obtained == NULL'.  'open_audio_device' cannot deal with 'desired' and 'obtained' pointing to the same block of memory but can deal with 'obtained == NULL'