Don't throw an exception in the notification handler if we're shutting down

OCaml calls WSACleanup in an at_exit handler. If we try to send a
notification after that we get an error and throw an exception in the
signal handler. This causes a segfault.

Fixes #59.
Initialise notification byte

Avoids memory checker warning (drmemory) about reading an uninitialised
byte.
Merge pull request #61 from talex5/win-shutdown-fix

Don't throw an exception in the notification handler if we're shutting down