Make the count of bytes read from the file 64 bits.

We probably won't get files that big, but let's at least be prepared;
that squelches some compiler warnings.

Change-Id: Ia43ff78af7df63c7204c41d8331fea946de63116
Reviewed-on: https://code.wireshark.org/review/4015
Reviewed-by: Guy Harris <guy@alum.mit.edu>