jpeg2000dec: make cblk->length(inc)/data use consistent

Fixes Ticket2612
Reviewed-by: Nicolas BERTRAND <nicoinattendu@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/jpeg2000dec.c

@@ -671,17 +671,11 @@ static int jpeg2000_decode_packet(Jpeg2000DecoderContext *s,
         for (cblkno = 0; cblkno < nb_code_blocks; cblkno++) {
             Jpeg2000Cblk *cblk = prec->cblk + cblkno;
             if (   bytestream2_get_bytes_left(&s->g) < cblk->lengthinc
-                || sizeof(cblk->data) < cblk->lengthinc
+                || sizeof(cblk->data) < cblk->length + cblk->lengthinc + 2
             )
                 return AVERROR(EINVAL);
-            /* Code-block data can be empty. In that case initialize data
-             * with 0xFFFF. */
-            if (cblk->lengthinc > 0) {
-                bytestream2_get_bufferu(&s->g, cblk->data, cblk->lengthinc);
-            } else {
-                cblk->data[0] = 0xFF;
-                cblk->data[1] = 0xFF;
-            }
+
+            bytestream2_get_bufferu(&s->g, cblk->data + cblk->length, cblk->lengthinc);
             cblk->length   += cblk->lengthinc;
             cblk->lengthinc = 0;
         }