ccm: Destroy IV generator on crypter creation failure

Closes strongswan/strongswan#343.

src/libstrongswan/plugins/ccm/ccm_aead.c

@@ -417,6 +417,7 @@ ccm_aead_t *ccm_aead_create(encryption_algorithm_t algo,
 
 	if (!this->crypter)
 	{
+		this->iv_gen->destroy(this->iv_gen);
 		free(this);
 		return NULL;
 	}