avcodec/jpeg2000dec: Check cdx/y values more carefully

Some invalid values where not handled correctly in the later pixel
format matching code.
Fixes out of array accesses
Fixes Ticket2848

Found-by: Piotr Bandurski <ami_stuff@o2.pl>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/jpeg2000dec.c

@@ -273,7 +273,8 @@ static int get_siz(Jpeg2000DecoderContext *s)
         s->sgnd[i]   = !!(x & 0x80);
         s->cdx[i]    = bytestream2_get_byteu(&s->g);
         s->cdy[i]    = bytestream2_get_byteu(&s->g);
-        if (!s->cdx[i] || !s->cdy[i]) {
+        if (   !s->cdx[i] || s->cdx[i] == 3 || s->cdx[i] > 4
+            || !s->cdy[i] || s->cdy[i] == 3 || s->cdy[i] > 4) {
             av_log(s->avctx, AV_LOG_ERROR, "Invalid sample seperation\n");
             return AVERROR_INVALIDDATA;
         }