Zero initialize stack buffers so that strlen() can't overrun them

Found with Address Sanitizer in test VTK::RenderingVolumeOpenGL2Cxx-TestGPURayCastDepthPeelingBoxWidget.

Error was:

==61971==ERROR: AddressSanitizer: stack-buffer-overflow on address 0x7ffee20da410 at pc 0x00011e67a3f7 bp 0x7ffee20d9eb0 sp 0x7ffee20d9658
READ of size 257 at 0x7ffee20da410 thread T0
    #0 0x11e67a3f6 in wrap_strlen (libclang_rt.asan_osx_dynamic.dylib:x86_64+0x153f6)
    #1 0x10ee6d495 in vtkRenderWindowInteractor::SetKeySym(char const*) vtkRenderWindowInteractor.h:571
    #2 0x1145af7bd in vtkInteractorEventRecorder::Play() vtkInteractorEventRecorder.cxx:264
    #3 0x10db406d3 in TestGPURayCastDepthPeelingBoxWidget(int, char**) TestGPURayCastDepthPeelingBoxWidget.cxx:1439
    #4 0x10db2ec67 in main vtkRenderingVolumeOpenGL2CxxTests.cxx:284
    #5 0x7fff75b183d4 in start (libdyld.dylib:x86_64+0x163d4)