avformat/iff: simplify duration calculation

Fixes: signed integer overflow: 315680096256 * 134215943 cannot be represented in type 'long long'
Fixes: 48713/clusterfuzz-testcase-minimized-ffmpeg_dem_IFF_fuzzer-5886272312311808

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

libavformat/iff.c

@@ -384,7 +384,7 @@ static int read_dst_frame(AVFormatContext *s, AVPacket *pkt)
                 avio_skip(pb, 1);
             pkt->flags |= AV_PKT_FLAG_KEY;
             pkt->stream_index = 0;
-            pkt->duration = 588LL * s->streams[0]->codecpar->sample_rate / 44100;
+            pkt->duration = s->streams[0]->codecpar->sample_rate / 75;
             pkt->pos = chunk_pos;
 
             chunk_pos = avio_tell(pb);
@@ -397,7 +397,8 @@ static int read_dst_frame(AVFormatContext *s, AVPacket *pkt)
         case ID_FRTE:
             if (data_size < 4)
                 return AVERROR_INVALIDDATA;
-            s->streams[0]->duration = avio_rb32(pb) * 588LL * s->streams[0]->codecpar->sample_rate / 44100;
+            s->streams[0]->duration = avio_rb32(pb) * (uint64_t)s->streams[0]->codecpar->sample_rate / 75;
+
             break;
         }