sw_redlinehide: don't return COMPLETE_STRING from MapModelToView()

The anchor position of a fly frame can be on an empty node after the
last non-deleted text, and SwAnchoredObject::CheckCharRect()
needs to get a valid frame index for that.

This can be reproduced with ooo69593-1.odt.

Change-Id: I448bb49d64e72b0af24306bd7c45755aa631b57d
Reviewed-on: https://gerrit.libreoffice.org/65488
Tested-by: Jenkins
Reviewed-by: Michael Stahl <Michael.Stahl@cib.de>

sw/source/core/text/txtfrm.cxx

@@ -1145,7 +1145,7 @@ TextFrameIndex MapModelToView(MergedPara const& rMerged, SwTextNode const*const
         assert(nIndex <= pNode->Len());
         return TextFrameIndex(0);
     }
-    return TextFrameIndex(COMPLETE_STRING);
+    return TextFrameIndex(rMerged.mergedText.getLength());
 }
 
 } // namespace sw