sw_redlinehide: fix SwCursorShell::GetContentAtPos() redline code

This calls SwTextFrame::GetCharRect() on the frame where the mouse
cursor is, with SwPositions that may be in a unrelated SwTextNode.

Bogus positions now triggers asserts, so let's fix it.

Change-Id: Idedb335ffd79c299976ad91b683dfb30cd0ceac2

sw/source/core/crsr/crstrvl.cxx

@@ -1518,11 +1518,17 @@ bool SwCursorShell::GetContentAtPos( const Point& rPt,
 
                         if( pFieldRect && nullptr != ( pFrame = pTextNd->getLayoutFrame( GetLayout(), &aPt ) ) )
                         {
+                            // not sure if this should be limited to one
+                            // paragraph, or mark the entire redline; let's
+                            // leave it limited to one for now...
+                            sal_Int32 nStart;
+                            sal_Int32 nEnd;
+                            pRedl->CalcStartEnd(pTextNd->GetIndex(), nStart, nEnd);
                             //get bounding box of range
                             SwRect aStart;
-                            pFrame->GetCharRect(aStart, *pRedl->Start(), &aTmpState);
+                            pFrame->GetCharRect(aStart, SwPosition(*pTextNd, nStart), &aTmpState);
                             SwRect aEnd;
-                            pFrame->GetCharRect(aEnd, *pRedl->End(), &aTmpState);
+                            pFrame->GetCharRect(aEnd, SwPosition(*pTextNd, nEnd), &aTmpState);
                             if (aStart.Top() != aEnd.Top() || aStart.Bottom() != aEnd.Bottom())
                             {
                                 aStart.Left(pFrame->getFrameArea().Left());