Fix potential exception after unmounting
Fix potential exception after unmounting

handleBlur() calls deferFocusInput() if the date picker is open.
deferFocusInput() schedules a later setFocus() call, which didn't
make sure the input element is still mounted before focusing it,
yielding an unhandled exception.

Hacker0x01/react-datepicker,Hacker0x01/react-datepicker,Hacker0x01/react-datepicker