revert lavc/dvbsub_parser.c: fix intermittent DVB subtitles disappearing

Originally from:
6aaf617

references:
https://code.mythtv.org/trac/ticket/5978
I could not find the referenced FFmpeg bugs from the comment or MythTV trac
since FFmpeg has changed its bug tracker. However, I did find
https://samples.ffmpeg.org/ffmpeg-bugs/roundup/issue378/Issue378/
which plays back with subtitles fine.

As far as I can tell the change reduces out_size by len + 6 if that section is
at the end of p.  However, that data is all in the buffer, so I don't see how
that would fix anything.  If this does actually fix something a new FFmpeg bug
report should be created with a sample.

mythtv/external/FFmpeg/libavcodec/dvbsub_parser.c

@@ -124,11 +124,7 @@ static int dvbsub_parse(AVCodecParserContext *s,
             {
                 len = AV_RB16(p + 4);
 
-                // MythTV #5978 "<=" -> "<"
-                // This is unresolved ffmpeg issue 378, use
-                // their solution if/when this is fixed upstream.
-                //if (len + 6 <= p_end - p)
-                if (len + 6 < p_end - p)
+                if (len + 6 <= p_end - p)
                 {
                     out_size += len + 6;