avformat/qcp: Avoid negative nb_rates

Fixes: signed integer overflow: 2 * -1725947872 cannot be represented in type 'int'
Fixes: 29102/clusterfuzz-testcase-minimized-ffmpeg_dem_QCP_fuzzer-6726807632084992

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

libavformat/qcp.c

@@ -93,7 +93,8 @@ static int qcp_read_header(AVFormatContext *s)
     QCPContext    *c  = s->priv_data;
     AVStream      *st = avformat_new_stream(s, NULL);
     uint8_t       buf[16];
-    int           i, nb_rates;
+    int           i;
+    unsigned      nb_rates;
 
     if (!st)
         return AVERROR(ENOMEM);