SwCache::DeleteObj() may decide to shrink the cache, and then the
SwTextFrame::mnCacheIndex goes stale, because only
SwCacheObj::m_nCachePos is updated.

In this bugdoc, this can happen *inside* SwTextFrame::Format(), where
first it succeeds to find an existing SwTextLine, then some footnotes
anchored in this paragraph are moved around and formatted, creating new
SwTextLines, and SwCache::DeleteObj is called.  Later, any access of the
original frame's SwTextLine fails to find it and eventually some null
pointer crash happens.

Newly added SwTextLine::UpdateCachePos() requires that SwTextFrame lives
longer than its SwTextLine; there was another problem with that, because
SwTextFrame::FormatEmpty() was throwing away the mnCacheIndex, which
could then cause a second SwTextLine to be created for the same
SwTextFrame, and the first one is not deleted until it falls to the
bottom of the LRU list.

Apprently for this particular document the problem didn't happen before
commit 3d37463e and/or
commit 31ae7509 but that is mostly luck.

Reviewed-on: https://gerrit.libreoffice.org/73047
Tested-by: Jenkins
Reviewed-by: Michael Stahl <Michael.Stahl@cib.de>
(cherry picked from commit 1424d51a)

Change-Id: I7bef1b340a453d6dd44d51a1dc69ee5fd0b697db