avformat/lrcenc: Avoid reading packet padding

(Everything would be fine if the packet was properly padded.)

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>

libavformat/lrcenc.c

@@ -105,7 +105,7 @@ static int lrc_write_packet(AVFormatContext *s, AVPacket *pkt)
                     size--;
                 next_line++;
             }
-            if(line[0] == '[') {
+            if (size && line[0] == '[') {
                 av_log(s, AV_LOG_WARNING,
                        "Subtitle starts with '[', may cause problems with LRC format.\n");
             }