stop duplicate definition of lcl_popOrZero()

This renames it to popOrZero() in node.hxx. Also one more call site
found in mathmlimport.hxx.

Change-Id: I3f0281315146347280d0836ceadcfdf9c80fa166
Reviewed-on: https://gerrit.libreoffice.org/12310
Reviewed-by: Noel Grandin <noelgrandin@gmail.com>
Tested-by: Noel Grandin <noelgrandin@gmail.com>