[REVIEW 3.4 3.5] fdo#40261: Fix crash in XML Form Document

Hi,

Can you please review a fix for:

https://bugs.freedesktop.org/show_bug.cgi?id=40261

It targets both libreoffice-3-4, and libreoffice-3-5, please review and
push the following there:

http://cgit.freedesktop.org/libreoffice/core/commit/?id=8912cf30755a2a19d50acc3bb0f5352506638fad

The commit log explains the details; the crash happened when somehow the
vector holding the indexes and the unordered_map holding the actual data
got out of sync.  I did not bother trying to fix it by getting them in
sync again, and instead cleaned up that mess.  Due to that, the patch is
a bit longer; but because it was crashing there always, making the XML
Form Document unusable, I hope just a basic code review + brief test
will do as a review.

Thank you,
Kendy