avformat/matroskadec: Check pre_ns for overflow

Fixes: signed integer overflow: -3483479120376300096 - 7442323944145700864 cannot be represented in type 'long'
Fixes: 383187489/clusterfuzz-testcase-minimized-ffmpeg_dem_WEBM_DASH_MANIFEST_fuzzer-4561470580391936

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

libavformat/matroskadec.c

@@ -4618,9 +4618,10 @@ static int64_t webm_dash_manifest_compute_bandwidth(AVFormatContext *s, int64_t
             // The prebuffer ends in the last Cue. Estimate how much data was
             // prebuffered.
             pre_bytes = desc_end.end_offset - desc_end.start_offset;
-            pre_ns = desc_end.end_time_ns - desc_end.start_time_ns;
+            if (desc_end.end_time_ns <= desc_end.start_time_ns ||
-            if (pre_ns <= 0)
+                desc_end.end_time_ns - (uint64_t)desc_end.start_time_ns > INT64_MAX)
                 return -1;
+            pre_ns = desc_end.end_time_ns - desc_end.start_time_ns;
             pre_sec = pre_ns / nano_seconds_per_second;
             prebuffer_bytes +=
                 pre_bytes * ((temp_prebuffer_ns / nano_seconds_per_second) / pre_sec);