avcodec/jpeg2000dec: Check that step_x/y are valid before use in JPEG2000_PGOD_PCRL

Fixes: CID1322305 and CID1322304
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/jpeg2000dec.c

@@ -1244,6 +1244,10 @@ static int jpeg2000_decode_packets_po_iteration(Jpeg2000DecoderContext *s, Jpeg2
                 step_y = FFMIN(step_y, rlevel->log2_prec_height + reducedresno);
             }
         }
+        if (step_x >= 31 || step_y >= 31){
+            avpriv_request_sample(s->avctx, "PCRL with large step");
+            return AVERROR_PATCHWELCOME;
+        }
         step_x = 1<<step_x;
         step_y = 1<<step_y;