The mpServerFont member of a ImplServerFontEntry must not be deleted while the
ImplServerFontEntry still exists

see also 39cbce55 for another reason a crash in
the same place can happen.  Its impossible from traces in crashes before
39cbce55 was fixed to distinguish those crashes
from this crash.

This crash is a regression due to 7a416820
where we went from modifying pServerFont in X11SalGraphics::setFont directly to
modifying it/a-different-one indirectly via ImplServerFontEntry

The various font caches and font thing lifecycles of LibreOffice are somewhat
confusing.

This crash had eluded me for years, to reproduce:
insert->special chars->select a font with loads of glyphs, i.e. "AR PL UKai CN"
click on the first row of glyphs and hold down page-down until you hit the
bottom, then page-up until you hit the top. Pre patch it won't survive the
whole down+up (and valgrind will moan quite a bit)

Change-Id: Ifde0cb375f487c556b04a640d77765a7dc2f0913