check for channels<=0 and print a reasonable error message

Originally committed as revision 8857 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/mpegaudio.c

@@ -70,8 +70,10 @@ static int MPA_encode_init(AVCodecContext *avctx)
     int i, v, table;
     float a;
 
-    if (channels > 2)
+    if (channels <= 0 || channels > 2){
+        av_log(avctx, AV_LOG_ERROR, "encoding %d channel(s) is not allowed in mp2\n", channels);
         return -1;
+    }
     bitrate = bitrate / 1000;
     s->nb_channels = channels;
     s->freq = freq;