avcodec/dirac_parser: Fix overflow in dts

Fixes: signed integer overflow: -2147483648 - 1 cannot be represented in type 'int'
Fixes: 15568/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_DIRAC_fuzzer-5634719611355136

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>
(cherry picked from commit 549fcba8fc)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/dirac_parser.c

@@ -204,7 +204,7 @@ static int dirac_combine_frame(AVCodecParserContext *s, AVCodecContext *avctx,
         if (parse_timing_info && pu1.prev_pu_offset >= 13) {
             uint8_t *cur_pu = pc->buffer +
                               pc->index - 13 - pu1.prev_pu_offset;
-            int pts = AV_RB32(cur_pu + 13);
+            int64_t pts = AV_RB32(cur_pu + 13);
             if (s->last_pts == 0 && s->last_dts == 0)
                 s->dts = pts - 1;
             else