Fix index error in dicomdump.
Fix index error in dicomdump.

When dicomdump was used with "-k" to dump a tag path (i.e. a value within
a sequence), then there was a potential for a crash if the input was
multiple files within a DICOM series.

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