Investigate "expected loop to be an instance of Coolio::Loop" error

[repeatedly]

From fluentd issue.

https://github.com/fluent/fluentd/issues/703

Error log is here: https://github.com/shoichikaji/fluentd-error/blob/master/log/fluentd-only-error-coolio-1.4.2.log

[repeatedly]

I think adding nil check fixes attach related error. This assumes passing nil to loop argument doesn't happen on normal situations.

https://github.com/tarcieri/cool.io/blob/5446a5e44f508ce1589a33e7ae61464530f857c2/ext/cool.io/watcher.h#L13

[repeatedly]

For detach related error, we need to check when this error happens.

https://github.com/tarcieri/cool.io/blob/5446a5e44f508ce1589a33e7ae61464530f857c2/ext/cool.io/watcher.h#L31 or https://github.com/tarcieri/cool.io/blob/5446a5e44f508ce1589a33e7ae61464530f857c2/ext/cool.io/watcher.h#L36