Replace always true condition with a more likely one

indexOf is always lower than string length since "not found" is
encoded as -1. Original code works correctly anyway since the
"not found" case lead to copying the whole string, although this
copy is completely unnecessary.

Change-Id: Ic5dd995dd0c3f974c77b5bf209ad5e994b044385
Reviewed-on: https://gerrit.libreoffice.org/67320
Tested-by: Jenkins
Reviewed-by: Matteo Casalin <matteo.casalin@yahoo.com>

sw/source/filter/ww8/ww8atr.cxx

@@ -2866,9 +2866,9 @@ void AttributeOutputBase::TextField( const SwFormatField& rField )
                         if (pDocInfoField != nullptr)
                         {
                             OUString sFieldname = pDocInfoField->GetFieldName();
-                            sal_Int32 nIndex = sFieldname.indexOf(':');
 
-                            if (nIndex != sFieldname.getLength())
+                            const sal_Int32 nIndex = sFieldname.indexOf(':');
+                            if (nIndex >= 0)
                                 sFieldname = sFieldname.copy(nIndex + 1);
 
                             sStr = "\"" + sFieldname + "\"";