I was valgrinding a scene with a flickable and i come to see
that in some ocassions we use lastPos before initializing it
in QQuickFlickablePrivate::viewportAxisMoved

Change-Id: Ibfeeefc00f62493163d5d44f676bec79a4c004c2
Reviewed-by: Matthew Vogt <matthew.vogt@qinetic.com.au>