fdo#81380: LO crashes while opening the document

LO crashes while opening the document because PropertySet interface
is null and check for this condition was missing, so added the check.

Change-Id: I04dd873ea0923a662e27f9eb2013cde31edc912b
Reviewed-on: https://gerrit.libreoffice.org/10379Reviewed-by: Luboš Luňák <l.lunak@collabora.com>
Tested-by: Luboš Luňák <l.lunak@collabora.com>

writerfilter/source/dmapper/PropertyMap.cxx

@@ -549,7 +549,8 @@ void SectionPropertyMap::ApplyBorderToPageStyles(
             sal_uInt32 nLineWidth = 0;
             if (m_pBorderLines[nBorder])
                 nLineWidth = m_pBorderLines[nBorder]->LineWidth;
-            SetBorderDistance( xFirst, aMarginIds[nBorder], aBorderDistanceIds[nBorder],
+            if(xFirst.is())
+                SetBorderDistance( xFirst, aMarginIds[nBorder], aBorderDistanceIds[nBorder],
                   m_nBorderDistances[nBorder], nOffsetFrom, nLineWidth );
             if(xSecond.is())
                 SetBorderDistance( xSecond, aMarginIds[nBorder], aBorderDistanceIds[nBorder],