avcodec/dvbsubdec: Use ff_set_dimensions()

Fixes: signed integer overflow: 65313 * 65313 cannot be represented in type 'int'
Fixes: 15740/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_DVBSUB_fuzzer-5641749164195840

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Reviewed-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/dvbsubdec.c

@@ -1578,8 +1578,9 @@ static int dvbsub_parse_display_definition_segment(AVCodecContext *avctx,
     display_def->width   = bytestream_get_be16(&buf) + 1;
     display_def->height  = bytestream_get_be16(&buf) + 1;
     if (!avctx->width || !avctx->height) {
-        avctx->width  = display_def->width;
-        avctx->height = display_def->height;
+        int ret = ff_set_dimensions(avctx, display_def->width, display_def->height);
+        if (ret < 0)
+            return ret;
     }
 
     if (info_byte & 1<<3) { // display_window_flag