possible fix for ALSA #750 (crash when connect/disconnect devices)

[RossBencina]

possible fix for #750

(simphone project submitted the attached patch by email. requires rework and testing to make it into a viable PR, but perhaps it helps someone get started).

simphone wrote:

the patch has mostly to do with error recovery in the (blocking) alsa host api. comments inside the patch explain what was changed and why it had to be changed. i won't go into great detail here, except to say that this patch now finally allows receiving an "unanticipated host error" when an audio device is unplugged from a linux system, just as it happens with other host apis (for example, on windows). otherwise, such changes usually go undetected with alsa.

401_portaudio_alsa.diff.txt