Fix exivextractor crash with malformed files

Summary:
Prevent a segfault by explicitly checking everywhere
for valid entries.

BUG: 405210

Reviewers: bruns, ngraham

Reviewed By: bruns, ngraham

Subscribers: kde-frameworks-devel, #baloo

Tags: #frameworks, #baloo

Differential Revision: https://phabricator.kde.org/D20877