Pass RuleWithActions::executeTransformation arguments by reference

- This function already expects these arguments not to be null pointers,
  doesn't validate them and just dereference them.
- In order to make this explicit and enforced by the compiler, they're
  now passed as references.

headers/modsecurity/rule_with_actions.h

@@ -161,9 +161,9 @@ class RuleWithActions : public Rule {
         const actions::transformations::Transformation &a,
         std::string &value,
         const Transaction *trans,
-        TransformationResults *ret,
-        std::string *path,
-        int *nth) const;
+        TransformationResults &ret,
+        std::string &path,
+        int &nth) const;
 
     /* actions */
     actions::Action *m_disruptiveAction;