Prior to this patch, a read error in snd_pcm_readi would still be treated as a
nominal result when constructing a voice frame from the expected data. Since
the value returned is negative, as opposed to the number of samples read,
this could result in a crash. With this patch, we now return a null frame
when a read error is detected.

Note that the patch on ASTERISK-21329 was modified slightly for this commit,
in that we bail immediately on detecting the read error, rather than bypassing
the construction of the voice frame.

(closes issue ASTERISK-21329)
Reported by: Keiichiro Kawasaki
patches:
  chan_alsa.diff uploaded by kawasaki (License 6489)
........

Merged revisions 385633 from http://svn.asterisk.org/svn/asterisk/branches/1.8


git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/11@385634 65c4cc65-6c06-0410-ace0-fbb531ad65f3