Wrap tvb_get_ptr() calls in the CRC routines.

That doesn't eliminate them, but at least it encapsulates them.

Change-Id: I78f0202cb7d2eb86e2dce220b2b97acc256d1e42
Reviewed-on: https://code.wireshark.org/review/3489
Reviewed-by: Guy Harris <guy@alum.mit.edu>