xwma: Validate channels and bits_per_coded_sample.

This prevents a SIGFPE later on.

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
CC: libav-stable@libav.org

libavformat/xwma.c

@@ -115,6 +115,17 @@ static int xwma_read_header(AVFormatContext *s)
         }
     }
 
+    if (!st->codec->channels) {
+        av_log(s, AV_LOG_WARNING, "Invalid channel count: %d\n",
+               st->codec->channels);
+        return AVERROR_INVALIDDATA;
+    }
+    if (!st->codec->bits_per_coded_sample) {
+        av_log(s, AV_LOG_WARNING, "Invalid bits_per_coded_sample: %d\n",
+               st->codec->bits_per_coded_sample);
+        return AVERROR_INVALIDDATA;
+    }
+
     /* set the sample rate */
     avpriv_set_pts_info(st, 64, 1, st->codec->sample_rate);