openssl: Make sure to release the functional ENGINE reference

The functional reference created by ENGINE_init() was never released,
only the structural one created by ENGINE_by_id().  The functional
reference includes an implicit structural reference, which is also
released by ENGINE_finish().

Closes strongswan/strongswan#119.

Author: tobiasbrunner

src/libstrongswan/plugins/openssl/openssl_plugin.c

@@ -436,21 +436,21 @@ static private_key_t *openssl_private_key_connect(key_type_t type,
 		ENGINE_free(engine);
 		return NULL;
 	}
+	ENGINE_free(engine);
 	if (!login(engine, keyid))
 	{
 		DBG1(DBG_LIB, "login to engine '%s' failed", engine_id);
-		ENGINE_free(engine);
+		ENGINE_finish(engine);
 		return NULL;
 	}
 	key = ENGINE_load_private_key(engine, keyname, NULL, NULL);
+	ENGINE_finish(engine);
 	if (!key)
 	{
 		DBG1(DBG_LIB, "failed to load private key with ID '%s' from "
 			 "engine '%s'", keyname, engine_id);
-		ENGINE_free(engine);
 		return NULL;
 	}
-	ENGINE_free(engine);
 
 	switch (EVP_PKEY_base_id(key))
 	{