[Libreoffice] Review requests for mailmerge to email fixes

There's a cluster of messy mail merge related bugs.

https://bugs.freedesktop.org/show_bug.cgi?id=32553
https://bugs.freedesktop.org/show_bug.cgi?id=31792
https://bugs.freedesktop.org/show_bug.cgi?id=34187

and I'd like backport three concrete and fairly trivial fixes which
address the reproducible parts to 3-3-2

Firstly, the hang.
http://cgit.freedesktop.org/libreoffice/writer/commit/?id=7482695994c5f6bf9995748944e38d2792aaed92
nNodeCnt is a ULONG, and is therefore unsigned, the loop counts down to
0, if nNodeCnt is less than the value which gets subtracted from it, it
natually wraps around and leaps off into hyperspace, hanging or
crashing. Safest fix IMO is to simply set it to 1 in this case so that
at the end of the loop the -- makes it hit 0 and loop ends.

Secondly, sending an email with an attached .odt. We're looking up the
mimetype of the odt by finding what filter handles .odt, sadly we now
have *two* filters that handle suffixes of .odt, i.e. the flat xml
filter as well, and that has no mime-type set in the filter config ->
epic fail. Simplest fix is to pull the known export filter directly
rather than hope for the best off ".odt"
http://cgit.freedesktop.org/libreoffice/writer/commit/?id=5c4a455137ae1fc74cdf88fe6d4d315b1948b4c0

Thirdly, send an email with an attached .pdf won't work unless mailmerge
was used with html or txt beforehand, which is apparently a similar
problem to two in that looking up the export filter for .pdf by what can
export a ".pdf" isn't getting the expected filter (possibly getting a
calc or draw .pdf exporter or something, didn't bother looking deeper)
http://cgit.freedesktop.org/libreoffice/writer/commit/?id=97bce7aad1c3a6f3fd23a3380c79ca9c1515624c

C.