Only call destroy on a client when it is not already being destroyed

Adds a check in the error listener on the client in the pool, to
prevent calling destroy on a client when it is already being
destroyed.
Without this check, if an error occurs during the ending of the
stream, such as a timeout, the client is never removed from
the pool and weird things happen.
Merge pull request #620 from Jarvix/timeout_error_fix

Pool: Only call destroy on a client when it is not already being destroyed