D15993: [PositionCodec] Avoid crash in case of corrupt data

[Nathaniel Graham]

ngraham added a reviewer: Baloo.

REPOSITORY
  R293 Baloo

REVISION DETAIL
  https://phabricator.kde.org/D15993

To: bruns, #frameworks, poboiko, ngraham, #baloo
Cc: kde-frameworks-devel, #baloo, ashaposhnikov, michaelh, astippich, spoorun, 
ngraham, bruns, abrahams