sw: fix crash in sw_unoapi due to calling wrong SwTxtPortion ctor

Crash in sw.SwXTextRange because the SwTxtPortion copy ctor is
invoked, which copies the portion type POR_LAY, but SwTxtPortion
must have POR_TXT.

(hilarious regression from 2db379e2)

Change-Id: I781191a60dafeba2257edf01699fafae78b45783

sw/source/core/text/porlay.cxx

@@ -139,7 +139,7 @@ SwLinePortion *SwLineLayout::Insert( SwLinePortion *pIns )
     {
         if( GetLen() )
         {
-            pPortion = new SwTxtPortion(*this);
+            pPortion = new SwTxtPortion(*static_cast<SwLinePortion*>(this));
             if( IsBlinking() && pBlink )
             {
                 SetBlinking( false );
@@ -161,7 +161,7 @@ SwLinePortion *SwLineLayout::Append( SwLinePortion *pIns )
     // First attribute change: copy mass and length from *pIns into the first
     // text portion
     if( !pPortion )
-        pPortion = new SwTxtPortion(*this);
+        pPortion = new SwTxtPortion(*static_cast<SwLinePortion*>(this));
     // Call with scope or we'll end up with recursion!
     return pPortion->SwLinePortion::Append( pIns );
 }
@@ -184,7 +184,7 @@ SwMarginPortion *SwLineLayout::CalcLeftMargin()
     SwMarginPortion *pLeft = (GetPortion() && GetPortion()->IsMarginPortion()) ?
         static_cast<SwMarginPortion *>(GetPortion()) : 0;
     if( !GetPortion() )
-         SetPortion(new SwTxtPortion(*this));
+         SetPortion(new SwTxtPortion(*static_cast<SwLinePortion*>(this)));
     if( !pLeft )
     {
         pLeft = new SwMarginPortion( 0 );