tdf#66353: fix converting relative sw table formula back to box

The check was incorrect: it invariantly checked only first part
of formula, and thus gave wrong results when the relative represen-
tation of formula contained external reference in usual box form.

BTW, I don't know why there's hardcoded return of "A1" on error
in SwTableFormula::RelNmsToBoxNms? Isn't that dangerous to provide
an arbitrary valid result that may give unnoticed wrong calculation?

Change-Id: If17c0f9459c64e65a6c2b8ed4482ffee78f115b2
Reviewed-on: https://gerrit.libreoffice.org/15678Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>

sw/source/core/fields/cellfml.cxx

@@ -648,7 +648,7 @@ OUString SwTableFormula::ScanString( FnScanFormula fnFormula, const SwTable& rTb
             // JP 22.02.99: Linux compiler needs cast
             // JP 28.06.99: rel. BoxName has no preceding tablename!
             if( fnFormula != (FnScanFormula)&SwTableFormula::_SplitMergeBoxNm &&
-                m_sFormula.getLength()>1 && cRelIdentifier != m_sFormula[1] &&
+                m_sFormula.getLength()>(nStt+1) && cRelIdentifier != m_sFormula[nStt+1] &&
                 (nSeparator = m_sFormula.indexOf( '.', nStt ))>=0
                 && nSeparator < nEnd )
             {