[REVIEW 3-6-2 1/3] Crasher fix for xlsx import

[Kohei Yoshida]

I've cherry-picked

https://gerrit.libreoffice.org/gitweb?p=core.git;a=commitdiff;h=3ad52a6453ff612409123db29dc74a9071789aab

Just a minute ago to fix

https://bugs.freedesktop.org/show_bug.cgi?id=48366
https://bugs.freedesktop.org/show_bug.cgi?id=54609

the latter of which is a hard-hack.  The cherry-picking to the 3-6 
branch was done with my sign-off, and we need two more sign-offs to push 
this change into 3-6-2.

Kohei

-- 
Kohei Yoshida, LibreOffice hacker, Calc