avformat/sbgdec: Check that end is not before start

Fixes: signed integer overflow: -9223372036854775808 + -5279949906739200 cannot be represented in type 'long'
Fixes: 26908/clusterfuzz-testcase-minimized-ffmpeg_dem_SBG_fuzzer-6329610851319808

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Reviewed-by: Nicolas George <george@nsup.org>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavformat/sbgdec.c

@@ -1416,6 +1416,11 @@ static av_cold int sbg_read_header(AVFormatContext *avf)
     if (r < 0)
         goto fail;
 
+    if (script.end_ts != AV_NOPTS_VALUE && script.end_ts < script.start_ts) {
+        r = AVERROR_INVALIDDATA;
+        goto fail;
+    }
+
     st = avformat_new_stream(avf, NULL);
     if (!st)
         return AVERROR(ENOMEM);