Fix segfault if vtkImageWeightedSum's inputs don't match.

The vtkImageWeightedSumExecute method assumed that all of the inputs
had exactly the same scalar info, and would segfault if the scalar
size or the number of components didn't match.  Now, the filter fails
with an error instead of segfaulting.

Change-Id: I39fa336b35f25763a8383feaf578b7dbea9f7d13
Merge topic 'weighted-sum-segfault' into master

006d4d45 Fix segfault if vtkImageWeightedSum's inputs don't match.