[PositionCodec] Avoid crash in case of corrupt data
Summary:
getDifferentialVarInt* returns a nullptr in case of a decoding error.
See also D11825.
BUG: 367480
Reviewers: Frameworks, poboiko, ngraham, Baloo
Reviewed By: ngraham, Baloo
Subscribers: kde-frameworks-devel, Baloo
Tags: Frameworks, Baloo
Differential Revision: https://phabricator.kde.org/D15993