Fixing crash when opening .bib file

In certain situations, also depending on Linux distributions (varying Qt
and KDE Frameworks versions), opening a bibliography file can cause
crashes (segmentation faults) due to an invalid model or its underlying
data.

This commit changes the order of two instructions. Now first an internal
variable is properly set. Only then an update on the model is applied.

The largest part of this commit is a new test that reproducibly crashes
without the changed order of two instructions, but passes after the fix
has been applied.

This commit is a forward-port of commit 4aea6ed3 from
branch 'kbibtex/0.10'.

CCBUG: 433084
CCBUG: 453455