[Libreoffice] [REVIEW] Fix for fdo#37816

[Kohei Yoshida]

I'd like to cherry-pick the following commit

http://cgit.freedesktop.org/libreoffice/calc/commit/?id=34a1f66c5f69aba2cd6135895563cf569a2531eb

from master to the -3-4 branch.  This fixes a rather nasty crasher when
moving a field button inside a pivot table.  The bug is reported here:

https://bugs.freedesktop.org/show_bug.cgi?id=37816

This is a one-liner fix.

Review appreciated.

Kohei

-- 
Kohei Yoshida, LibreOffice hacker, Calc
<kyoshida at novell.com>