Fix illegal extent request in ImageThresholdConnectivity.

When asked for an output extent that was smaller than the input extent,
this filter would pass an illegal extent to GetScalarPointerForExtent().
It is possible that older versions of VTK would allow this, but in VTK 6
it raises an error.  It has a high probability of causing a segfault.

Change-Id: Ic024f9aa97335f1ba29b47be89a4e5be8195b00f
Merge topic 'imagethresholdconnectivity-segfault' into master

24265644 Fix illegal extent request in ImageThresholdConnectivity.