fix(pool): check that there is some connection in the pool before shrinking

fix(pool): check that there is some connection in the pool before shrinking

There is a race condition between shrinking the pool and closing an
expired connection which might result in attempting to pop from the
empty _pool deque. Add a check to prevent that.

The bug is not serious as it doesn't compromise the pool working but it causes
noise in the logging.

Close #1001