Wrap some tvb_get_ptr calls in crc32c_tvb_offset_calculate().

This eliminates some tvb_get_ptr calls from dissectors, as part of
trying to minimize if not eliminate its use in dissectors (it's a bit of
what's called an "attractive nuisance", in that users who don't
understand what it does - and doesn't! - do may use it in ways that are
unsafe, e.g. thinking you can use it to get a null-terminated string
from a packet).

It also eliminates the possibility of passing one length to
tvb_get_ptr() and another to crc32c_calculate().

Change-Id: I8a07168d0bc088b45d607e00c5bb1d98421ebc73
Reviewed-on: https://code.wireshark.org/review/3488
Reviewed-by: Guy Harris <guy@alum.mit.edu>