bitrate sanity check (fixes assertion failure)

Originally committed as revision 8293 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/wmaenc.c

@@ -36,6 +36,9 @@ static int encode_init(AVCodecContext * avctx){
     if(avctx->channels > MAX_CHANNELS)
         return -1;
 
+    if(avctx->bit_rate < 24*1000)
+        return -1;
+
     /* extract flag infos */
     flags1 = 0;
     flags2 = 1;