avcodec/dvdsub_parser: print message if packet is smaller than the packet size field

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
(cherry picked from commit bcc898dd)
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/dvdsub_parser.c

@@ -46,6 +46,8 @@ static int dvdsub_parse(AVCodecParserContext *s,
 
     if (pc->packet_index == 0) {
         if (buf_size < 2 || AV_RB16(buf) && buf_size < 6) {
+            if (buf_size)
+                av_log(avctx, AV_LOG_DEBUG, "Parser input %d too small\n", buf_size);
             return buf_size;
         }
         pc->packet_len = AV_RB16(buf);