Fix crash in vtkDICOMReader CanReadFile().
Fix crash in vtkDICOMReader CanReadFile().

This bug was introduced in May (after the 0.4.5 release, but before
the 0.5.0 release).  It is present in v0.5.0 and v0.5.1.

dgobbi/vtk-dicom,dgobbi/vtk-dicom,hendradarwin/vtk-dicom,dgobbi/vtk-dicom,hendradarwin/vtk-dicom,hendradarwin/vtk-dicom