Fix the failing tests on AIX Dashboard.

On AIX, the behavior of istringstream operator(>>) is undefined if the
stream already reached eof. All other dashboards can handle this
case properly with no action. The fix is to add eof check before this
operator in vtkVariant.cxx.

Change-Id: Icbd8cae9988d0109908a9614c6bf07a2c223ae93
Merge topic 'fix_AIX_tests' into master

246d6ead Fix the failing tests on AIX Dashboard.