Fix crash in vtkFixedSliceHandleRepresentation3D

If BuildRepresentation is being called without a renderer, DisplayToWorld()
was crashing. This was the case in the TestSetGet unit test.
Regression from f8fe12a648f2d08ea095beb093b43029062d256d

Change-Id: Ia9f8dc33a91af66fbda108a1832c242ccf6ebcf2
Merge topic 'fix-vtkfixedsizehandlerepresentation3D-crash' into master

7a2f6bf2 Fix crash in vtkFixedSliceHandleRepresentation3D