SwLinePortion::Truncate() called in
SwTextFormatter::FormatReset() delete the portion referred
by m_pFirstOfBorderMerge.

Change-Id: I68e134c86db617d37ea2efa09dad4409772146b5
Reviewed-on: https://gerrit.libreoffice.org/72300
Tested-by: Jenkins
Reviewed-by: Mark Hung <marklh9@gmail.com>