avcodec/dvbsubdec: check region dimensions

Fixes: 1408/clusterfuzz-testcase-minimized-6529985844084736
Fixes: integer overflow

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/targets/ffmpegSigned-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/dvbsubdec.c

@@ -24,6 +24,7 @@
 #include "bytestream.h"
 #include "internal.h"
 #include "libavutil/colorspace.h"
+#include "libavutil/imgutils.h"
 #include "libavutil/opt.h"
 
 #define DVBSUB_PAGE_SEGMENT     0x10
@@ -1127,6 +1128,7 @@ static int dvbsub_parse_region_segment(AVCodecContext *avctx,
     DVBSubObject *object;
     DVBSubObjectDisplay *display;
     int fill;
+    int ret;
 
     if (buf_size < 10)
         return AVERROR_INVALIDDATA;
@@ -1155,6 +1157,12 @@ static int dvbsub_parse_region_segment(AVCodecContext *avctx,
     region->height = AV_RB16(buf);
     buf += 2;
 
+    ret = av_image_check_size(region->width, region->height, 0, avctx);
+    if (ret < 0) {
+        region->width= region->height= 0;
+        return ret;
+    }
+
     if (region->width * region->height != region->buf_size) {
         av_free(region->pbuf);