BUG: Fix CheckForLabelVolumeValidity warning. Fixes #3776

This commit fixes a regression introduced in r23406 [1]
where calling CheckForLabelVolumeValidity with two volumes that have
the same dimension was returning a wrong warning, e.g. "IJKToRAS mismatch
at [0, 3] (-89.6207 != -89.6207)", "IJKToRAS mismatch
at [1, 1] (0.000542851 != 0.000542851)", and "IJKToRAS mismatch
at [2, 0] (4.32616e-09 != 4.32616e-09) IJKToRAS mismatch
at [2, 2] (0.0752821 != 0.0752821)", and "IJKToRAS mismatch
at [2, 3] (176.197 != 176.197)".

Thanks to "safir" for reporting the problem and suggesting a solution.

[1] http://viewvc.slicer.org/viewvc.cgi/Slicer4?view=revision&revision=23406

git-svn-id: http://svn.slicer.org/Slicer4/trunk@23488 3bd1e089-480b-0410-8dfb-8563597acbee