[Minor] Use the proper variable in debug

Found by: coverity scan

src/libutil/expression.c

@@ -1133,7 +1133,7 @@ rspamd_parse_expression (const gchar *line, gsize len,
 						"rewind stack; op: %s",
 				e->expression_stack->len,
 				operand_stack->len,
-				rspamd_expr_op_to_str (op));
+				rspamd_expr_op_to_str (op_stack));
 
 		if (op_stack != OP_OBRACE) {
 			elt.type = ELT_OP;