avformat/microdvddec: use 64bit for durations

Fixes: signed integer overflow: 7 - -2147483647 cannot be represented in type 'int'
Fixes: 28036/clusterfuzz-testcase-minimized-ffmpeg_dem_MICRODVD_fuzzer-5171698751766528

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

libavformat/microdvddec.c

@@ -65,12 +65,12 @@ static int64_t get_pts(const char *buf)
     return AV_NOPTS_VALUE;
 }
 
-static int get_duration(const char *buf)
+static int64_t get_duration(const char *buf)
 {
     int frame_start, frame_end;
 
     if (sscanf(buf, "{%d}{%d}", &frame_start, &frame_end) == 2)
-        return frame_end - frame_start;
+        return frame_end - (int64_t)frame_start;
     return -1;
 }