avformat/cache: Use int64_t to avoid int overflow in cache_read

Fixes an issue where an int64_t ffurl_seek return-value was being stored
in an int (32-bit) "r" variable, leading to integer overflow when seeking
into a large file (>2GB), and ultimately a "Failed to perform internal
seek" error mesage.

To test, try running `ffprobe 'cache:http://<something

>'` on a file that
is ~3GB large, whose moov atom is at the end of the file

Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>