Extended loplugin:ostr: hwpfilter

Change-Id: I1cd5359c192897d3d497eec977de10cedcc8f7b2
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/160120
Tested-by: Jenkins
Reviewed-by: Stephan Bergmann <stephan.bergmann@allotropia.de>

hwpfilter/source/hwpeq.cxx

@@ -534,9 +534,9 @@ static int next_token(OString &white, OString &token, std::istream *strm)
       token = buf;
     }
     if( token == "sub" || token == "from" )
-      token = "_";
+      token = "_"_ostr;
     if( token == "sup" || token == "to" )
-      token = "^";
+      token = "^"_ostr;
   }
   else if( IS_BINARY(ch) ) {
     do