tdf#124937 reset m_pFirstOfBorderMerge before truncate.

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>
(cherry picked from commit 96acebb7)
Reviewed-on: https://gerrit.libreoffice.org/72512Reviewed-by: Michael Stahl <Michael.Stahl@cib.de>

sw/source/core/text/itrform2.cxx

@@ -1861,6 +1861,7 @@ void SwTextFormatter::FeedInf( SwTextFormatInfo &rInf ) const
 
 void SwTextFormatter::FormatReset( SwTextFormatInfo &rInf )
 {
+    m_pFirstOfBorderMerge = nullptr;
     m_pCurr->Truncate();
     m_pCurr->Init();
     if( pBlink && m_pCurr->IsBlinking() )