avformat/rpl: Use 64bit for duration computation

Fixes: signed integer overflow: 24709512 * 88 cannot be represented in type 'int'
Fixes: 50993/clusterfuzz-testcase-minimized-ffmpeg_DEMUXER_fuzzer-6737973728641024

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

libavformat/rpl.c

@@ -279,7 +279,7 @@ static int rpl_read_header(AVFormatContext *s)
     error |= read_line(pb, line, sizeof(line));  // size of "helpful" sprite
     if (vst) {
         error |= read_line(pb, line, sizeof(line));  // offset to key frame list
-        vst->duration = number_of_chunks * rpl->frames_per_chunk;
+        vst->duration = number_of_chunks * (int64_t)rpl->frames_per_chunk;
     }
 
     // Read the index