alsa: fixed Segmentation fault due to dangling pointer left by snd_lib_error_set_hand...

alsa: fixed Segmentation fault due to dangling pointer left by snd_lib_error_set_handler() call which set callback pointer to ALSA during PaAlsa_Initialize call. Segmentation fault happens only if application dynamically unloaded PortAudio while still using ALSA through other means, then next ALSA error which causes invocation of ALSA error handling callbacks would cause Segmentation fault. Bug reported in: http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=563192, and re-announced by Phil Burk on 15.10.2010.