tdf#124722 sw_redlinehide: don't set negative SwTextFrame offsets

nLen may be larger than the master SwTextFrame, but its follow can't
have negative offset.

(regression from 0acde7514e666fc04805fd36503bd174162336ca)

Change-Id: I6177c748480cdf61e8f15a7032ba52d3ae2ea52c
Reviewed-on: https://gerrit.libreoffice.org/70816
Tested-by: Jenkins
Reviewed-by: Michael Stahl <Michael.Stahl@cib.de>
diff --git a/sw/source/core/text/txtfrm.cxx b/sw/source/core/text/txtfrm.cxx
index 66beb06..90541798 100644
--- a/sw/source/core/text/txtfrm.cxx
+++ b/sw/source/core/text/txtfrm.cxx
@@ -1856,7 +1856,7 @@
    assert(nLen != TextFrameIndex(COMPLETE_STRING));
    if (rFrame.IsFollow() && nPos < rFrame.GetOfst())
    {
        rFrame.ManipOfst( op(rFrame.GetOfst(), nLen) );
        rFrame.ManipOfst( std::max(TextFrameIndex(0), op(rFrame.GetOfst(), nLen)) );
    }
}