Don't attempt to create a voice frame on a read error

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

channels/chan_alsa.c

@@ -478,6 +478,13 @@ static struct ast_frame *alsa_read(struct ast_channel *chan)
 	} else if (r < 0) {
 		ast_log(LOG_ERROR, "Read error: %s\n", snd_strerror(r));
 	}
+
+	/* Return NULL frame on error */
+	if (r < 0) {
+		ast_mutex_unlock(&alsalock);
+		return &f;
+	}
+
 	/* Update positions */
 	readpos += r;
 	left -= r;