Squelch a compiler warning.

Some compilers complain that we're casting a 16-bit integer to a
pointer; making it a standard-size integer squelches the warning.

Change-Id: Iba0e767d7801afdc369b64a1cf050bf8db37da7b
Reviewed-on: https://code.wireshark.org/review/5264
Reviewed-by: Guy Harris <guy@alum.mit.edu>