amr: Set the codec->bit_rate field based on the last packet

This allows libavformat to guess an estimated duration for
amr files.

For streams with varying bit rates (or with silence descriptors
or "no frame" blocks) the guess is, of course, inaccurate.

libavformat/amr.c

@@ -153,6 +153,9 @@ static int amr_read_packet(AVFormatContext *s,
         return AVERROR(EIO);
     }
 
+    /* Both AMR formats have 50 frames per second */
+    s->streams[0]->codec->bit_rate = size*8*50;
+
     pkt->stream_index = 0;
     pkt->pos= avio_tell(s->pb);
     pkt->data[0]=toc;