avcodec/jpeg2000dec: Check Psot

Fixes out of array read

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/jpeg2000dec.c

@@ -469,6 +469,10 @@ static int get_sot(Jpeg2000DecoderContext *s, int n)
     /* Read TNSot but not used */
     bytestream2_get_byteu(&s->g);               // TNsot
 
+    if (Psot > bytestream2_get_bytes_left(&s->g) + n + 2) {
+        av_log(s->avctx, AV_LOG_ERROR, "Psot %d too big\n", Psot);
+        return AVERROR_INVALIDDATA;
+    }
     if (TPsot >= FF_ARRAY_ELEMS(s->tile[s->curtileno].tile_part)) {
         av_log(s->avctx, AV_LOG_ERROR, "TPsot %d too big\n", TPsot);
         return AVERROR_PATCHWELCOME;