File size >=2**31 wraps around

Reported via an email.

So, I'm not sure if this is a ffmpeg issue or a PyAV issue, but if you have a file size this is >=2**31, you'll get the wrong value.. either you'll get a size too small, or a negative number...

For example:

os.stat(fname)
posix.stat_result(st_mode=33060, st_ino=38303595, st_dev=234881028L, st_nlink=1, st_uid=502, st_gid=20, st_size=8163995316, st_atime=1412451616, st_mtime=1411500234, st_ctime=1411500234)
f = av.open(fname)
print f.size
-425939276
print f.size + 2**32 + 2**32
8163995316

so, this was built on MacOSX 10.7.5...