fix highlighting of '\0' in C mode

Summary:
before the second ' was red for the '\0' literal
now it is ok, just like '\123'

Test Plan: make && make test

Reviewers: dhaumann, vkrause, nibags

Reviewed By: dhaumann

Subscribers: kwrite-devel, kde-frameworks-devel

Tags: Kate, Frameworks

Differential Revision: https://phabricator.kde.org/D21467