Use nullptr for passing a null pointer to crc32

Summary:
I'm a bit unconvinced, wihtout it we get a warning if we enable -Wzero-as-null-pointer-constant
but using Z_NULL seems to be more "what the library wants"

If we don't want this, this would be a reason against enabling -Wzero-as-null-pointer-constant
for all the frameworks as i've suggested in the other review

Reviewers: mpyne

Reviewed By: mpyne

Subscribers: mpyne, Frameworks

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D10168