Properly handle situation if no resolver plugins are loaded

src/libcharon/plugins/ipseckey/ipseckey_plugin.c

@@ -60,7 +60,7 @@ METHOD(plugin_t, destroy, void,
 	{
 		lib->credmgr->remove_set(lib->credmgr, &this->cred->set);
 	}
-	this->res->destroy(this->res);
+	DESTROY_IF(this->res);
 	DESTROY_IF(this->cred);
 	free(this);
 }
@@ -87,8 +87,7 @@ plugin_t *ipseckey_plugin_create()
 
 	if (!this->res)
 	{
-		DBG1(DBG_CFG, "ipseckey_plugin: Failed to create"
-					  "a DNS resolver instance");
+		DBG1(DBG_CFG, "failed to create a DNS resolver instance");
 		destroy(this);
 		return NULL;
 	}

src/libstrongswan/resolver/resolver_manager.c

@@ -56,7 +56,11 @@ METHOD(resolver_manager_t, remove_resolver, void,
 METHOD(resolver_manager_t, create, resolver_t*,
 	private_resolver_manager_t *this)
 {
-	return this->constructor();
+	if (this->constructor)
+	{
+		return this->constructor();
+	}
+	return NULL;
 }
 
 METHOD(resolver_manager_t, destroy, void,