Verify the Pool connection is not nil before attempting to close it

[cquon]

This checks to make sure the Pool's connection is not nil before attempting to close it (previously it was not checking so could get a panic for a nil deference. If looking at the Pool connection initialization code in pool.go:

// NewPool creates a new connection pool for the given host
func NewPool(host Host, opts *ConnectOpts) (*Pool, error) {
	initialCap := opts.InitialCap
	if initialCap <= 0 {
		// Fallback to MaxIdle if InitialCap is zero, this should be removed
		// when MaxIdle is removed
		initialCap = opts.MaxIdle
	}

	maxOpen := opts.MaxOpen
	if maxOpen <= 0 {
		maxOpen = 1
	}

	conns := make([]*Connection, maxOpen)
	var err error
	for i := 0; i < opts.InitialCap; i++ {
		conns[i], err = NewConnection(host.String(), opts)
		if err != nil {
			return nil, err
		}
	}

	return &Pool{
		conns:   conns,
		pointer: -1,
		host:    host,
		opts:    opts,
		closed:  poolIsNotClosed,
	}, nil
}

It can be seen that if the opts.InitialCap < opts.MaxOpen, then we allocate a connection slice of size opts.MaxOpen where only opts.InitialCap are initially initialized. If we then close it later, it will cause the panic referred to above (can see stack trace in issue).

Addresses: #479

[CMogilko]

thanks for your contribution