Fix unitialized value warning, nullptr access

Summary:
getVarint32Ptr may return a nullptr (in case of an error), and also will
not set the value (size resp. n).

Signed-off-by: Stefan Brüns <stefan.bruens@rwth-aachen.de>

Test Plan: make

Reviewers: michaelh, Baloo

Reviewed By: michaelh, Baloo

Subscribers: Frameworks

Tags: Frameworks, Baloo

Differential Revision: https://phabricator.kde.org/D11825