avcodec/j2kenc: fix edge case when packet ends on 0xFF

ISO/IEC 15444-1:2002 (E), Annex B.10.1 "Bit-stuffing routine":

> The last byte in the packet header shall not be an 0xFF value (thus the
> single zero bit stuffed after a byte with 0xFF must be included even if
> the 0xFF would otherwise have been the last byte).

This matches the behavior seen in the jpeg2000 decoder (jpeg2000_flush),
and the behavior of openjpeg.

Sponsored-by: nxtedition AB
Signed-off-by: Niklas Haas <git@haasn.dev>

libavcodec/j2kenc.c

@@ -176,6 +176,8 @@ static void j2k_flush(Jpeg2000EncoderContext *s)
 {
     if (s->bit_index){
         s->bit_index = 0;
+        if (*s->buf == 0xff)
+            *(++s->buf) = 0;
         s->buf++;
     }
 }