[REVIEW 3-6] fdo#53938 Prevent crash on pivot table refresh on non-existent data source

[Kohei Yoshida]

I'd like the following two:

https://gerrit.libreoffice.org/gitweb?p=core.git;a=commitdiff;h=286ce144981aaa426622624302e296588abab1c2
https://gerrit.libreoffice.org/gitweb?p=core.git;a=commitdiff;h=6ad86f8cc036d01702e9d491874b2b3bde23fe77

backported to 3-6 in order to fix

https://bugs.freedesktop.org/show_bug.cgi?id=53938

It's a simple null pointer check, plus additional error checks.  Should 
be a safe change.

Thanks,

Kohei

-- 
Kohei Yoshida, LibreOffice hacker, Calc