avoid Asan runtime error: shift exponent -1 is negative

Summary:
Avoid doing a 1<<-1.

Test Plan:
Before: frameworks/ktexteditor/src/completion/katecompletionconfig.cpp:195:33: runtime error: shift exponent -1 is negative
after: silence

Reviewers: Kate, Frameworks, mwolff

Reviewed By: Kate, mwolff

Subscribers: mwolff, brauch

Tags: Kate, Frameworks

Differential Revision: https://phabricator.kde.org/D11811