Don't crash if alsa_snd_pcm_poll_descriptors fails

[daschuer]

This fixes #268

This patch was original proposed by Chameleon here:
https://portaudio.music.columbia.narkive.com/3V9hsUak/pa-linux-alsa-c-3636-assertion-failed-with-hack-fix

[philburk]

I will review the rest in more detail.

[philburk]

This has been discussed and approved by several people on the mail list.

[RossBencina]

I don't feel qualified to comment on this.

[philburk]

I faked an xrun in pa_linux_alsa.c at line 3645 with:

static int count = 0;
if( ret != self->nfds || ((count++ % 173) == 150)) // FIXME fake an error

Then I tried running the test paex_read_write_wire and it failed.
But the test aborts when it gets an xrun.
So I modified the test to just print a message when ReadStream or WriteStream return an error.
Now for every xrun I trigger in pa_alsa I get a flood of error returns.

[daschuer]

Done

[philburk]

Fixes #268

[antiochtech]

im still getting the error so i suspect my project is still using the old version. its a python project using the raspbian package python3-pyaudio. ive followed the installation recommendations for my device (https://github.com/PortAudio/portaudio/wiki/Platforms_RaspberryPi and http://portaudio.com/docs/v19-doxydocs/compile_linux.html), but i suspect theres another step?