avformat/jacosubenc: Fix writing extradata

The terminating '\0' is no longer included in the size of
the extradata output by the demuxer since commit
36e61e24e7.
E.g. if one remuxes the JACOsub sample JACOsub_capability_tester.jss
from the FATE suite, one receives a file not recognized as JACOsub
before this patch.

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

libavformat/jacosubenc.c

@@ -24,7 +24,7 @@ static int jacosub_write_header(AVFormatContext *s)
     const AVCodecParameters *par = s->streams[0]->codecpar;
 
     if (par->extradata_size) {
-        avio_write(s->pb, par->extradata, par->extradata_size - 1);
+        avio_write(s->pb, par->extradata, par->extradata_size);
     }
     return 0;
 }