nsvdec: fix division by 0

Fixes CID717749
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/nsvdec.c

@@ -653,7 +653,10 @@ null_chunk_retry:
                 if (bps != 16) {
                     av_dlog(s, "NSV AUDIO bit/sample != 16 (%d)!!!\n", bps);
                 }
-                bps /= channels; // ???
+                if(channels)
+                    bps /= channels; // ???
+                else
+                    av_log(s, AV_LOG_WARNING, "Channels is 0\n");
                 if (bps == 8)
                     st[NSV_ST_AUDIO]->codec->codec_id = AV_CODEC_ID_PCM_U8;
                 samplerate /= 4;/* UGH ??? XXX */