tdf#93600: Avoid crash on entering '=' as a cell value

To match formulas, 'formula is' condition should be used instead

Change-Id: I37a9a2680c49bffcb8b25a11faedc1421cfa2d00
Reviewed-on: https://gerrit.libreoffice.org/18126Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Markus Mohrhard <markus.mohrhard@googlemail.com>
Tested-by: Markus Mohrhard <markus.mohrhard@googlemail.com>
Reviewed-on: https://gerrit.libreoffice.org/18159Reviewed-by: Thorsten Behrens <Thorsten.Behrens@CIB.de>

sc/source/ui/condformat/condformatdlgentry.cxx

@@ -313,7 +313,7 @@ IMPL_LINK(ScConditionFrmtEntry, OnEdChanged, Edit*, pEdit)
     boost::scoped_ptr<ScTokenArray> ta(aComp.CompileString(aFormula));
 
     // Error, warn the user
-    if( ta->GetCodeError() )
+    if( ta->GetCodeError() || ( ta->GetLen() == 0 ) )
     {
         pEdit->SetControlBackground(COL_LIGHTRED);
         maFtVal->SetText(ScGlobal::GetRscString(STR_VALID_DEFERROR));