BUG: Fix InsertAndMergeExtent bug introduced in earlier commit.

Fix a bug that could cause delete [] to be called on an unallocated
pointer.  This bug was introduced in a945dc69, prior to VTK 5.10.

Backport of db0edf0a from master to release.

Change-Id: I3e0d5b432d43aed43b3bd2fe755d40d7f1c0b90e