avcodec/libmp3lame: do not attempt to flush lame if no data was input

this prevents the creation of a packet even though no single sample has ever
been input, which some confusion in the timestamp generation
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/libmp3lame.c

@@ -208,6 +208,8 @@ static int mp3lame_encode_frame(AVCodecContext *avctx, AVPacket *avpkt,
         default:
             return AVERROR_BUG;
         }
+    } else if (!s->afq.frame_alloc) {
+        lame_result = 0;
     } else {
         lame_result = lame_encode_flush(s->gfp, s->buffer + s->buffer_index,
                                         s->buffer_size - s->buffer_index);