Do not fail DVB sub decoding because of a few padding bytes

Instead of returning an error when bytes are left over, just return
the number of actually used bytes as other decoders do.
Instead add a special case so an error will be returned when none
of the data looks valid to avoid making debugging a pain.
Signed-off-by: Janne Grunau <janne-ffmpeg@jannau.net>
(cherry picked from commit 4a72765a)

libavcodec/dvbsubdec.c

@@ -1423,7 +1423,7 @@ static int dvbsub_decode(AVCodecContext *avctx,
 
 #endif
 
-    if (buf_size <= 2)
+    if (buf_size <= 2 || *buf != 0x0f)
         return -1;
 
     p = buf;
@@ -1467,12 +1467,7 @@ static int dvbsub_decode(AVCodecContext *avctx,
         p += segment_length;
     }
 
-    if (p != p_end) {
-        av_dlog(avctx, "Junk at end of packet\n");
-        return -1;
-    }
-
-    return buf_size;
+    return p - buf;
 }