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 96acebb72211b4718eb3038c427df37b55b17b0b)
Reviewed-on: https://gerrit.libreoffice.org/72512
Reviewed-by: Michael Stahl <Michael.Stahl@cib.de>
diff --git a/sw/qa/core/data/doc/tdf124937.doc b/sw/qa/core/data/doc/tdf124937.doc
new file mode 100644
index 0000000..4a3f91f
--- /dev/null
+++ b/sw/qa/core/data/doc/tdf124937.doc
Binary files differ
diff --git a/sw/source/core/text/itrform2.cxx b/sw/source/core/text/itrform2.cxx
index 473ed0a..a0778ad 100644
--- a/sw/source/core/text/itrform2.cxx
+++ b/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() )