carefully crafted cache file sizes can trigger assert

In read_file_to_str() we do

  if ((uint64_t)(statbuf.st_size)+1 > SIZE_T_CEILING)
    return NULL;

  string = tor_malloc((size_t)(statbuf.st_size+1));

So a remote attacker who can give you a combination of cached blobs such that one of your files becomes exactly SIZE_T_CEILING bytes (just 2 gigs on a 32 bit platform) could cause your Tor to consistently assert on start.

The fix is to check >= instead of >.

Reported by doors.