Fix memory leak when GateKeeperProxy.verify() returns

After verify() calls verifyChallenge(), the caller acquires the ownership of
returned memory block pointed by *auth_token.
However, the current implementation directly returns and lost the reference
of auth_token without freeing it from heap memory.

This patch solves this problem by explicitly deleting the auth_token array.

Change-Id: I6cfe8427174aa36fbb208e2fff8904095f468ec6