Free rend_data and intro_key when extra intro circs become general-purpose

  o Minor bugfixes:

    - When a hidden service turns an extra service-side introduction

      circuit into a general-purpose circuit, free the rend_data and

      intro_key fields first, so they won't be leaked if the circuit

      is cannibalized for use as another service-side introduction

      circuit.  Bugfix on 0.2.1.7-alpha; fixes bug 4251.

      log_info(LD_CIRC|LD_REND, "We have just finished an introduction "

               "circuit, but we already have enough. Redefining purpose to "

               "general; leaving as internal.");

      TO_CIRCUIT(circuit)->purpose = CIRCUIT_PURPOSE_C_GENERAL;

      {

        rend_data_t *rend_data = circuit->rend_data;

        circuit->rend_data = NULL;

        rend_data_free(rend_data);

      }

      {

        crypto_pk_env_t *intro_key = circuit->intro_key;

        circuit->intro_key = NULL;

        crypto_free_pk_env(intro_key);

      }

      circuit_has_opened(circuit);

      return;

    }