[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