In vtkXMLDataReader, commit c3a86fd5 added an exit-with-error when multiple
data arrays in point or cell data had the same name. This caused issues with
reading xml input data that had data arrays that had the same name, but
differed by timestep (see Issue #0015723). This commit reverts that change,
and instead guards against reading vtkDataArrays that are uninitialized
(which also fixes the error reported in Issue #0015525). A test for both of
these cases has also been added.