avcodec/dirac_parser: do not offset AV_NOPTS_OFFSET

Fixes: signed integer overflow: -9223372036854775807 - 48000 cannot be represented in type 'long long'
Fixes: 26521/clusterfuzz-testcase-minimized-ffmpeg_dem_DIRAC_fuzzer-5635536506847232

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Reviewed-by: Lynne <dev@lynne.ee>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/dirac_parser.c

@@ -215,7 +215,7 @@ static int dirac_combine_frame(AVCodecParserContext *s, AVCodecContext *avctx,
             int64_t pts = AV_RB32(cur_pu + 13);
             if (s->last_pts == 0 && s->last_dts == 0)
                 s->dts = pts - 1;
-            else
+            else if (s->last_dts != AV_NOPTS_VALUE)
                 s->dts = s->last_dts + 1;
             s->pts = pts;
             if (!avctx->has_b_frames && (cur_pu[4] & 0x03))