js_stream: prevent abort if isalive doesn't exist

Attempting to check IsAlive() on a JSStream before the isAlive()
callback can be set in JS causes a CHECK to fail in MakeCallback.
Instead return false if the callback hasn't been set.

PR-URL: https://github.com/nodejs/node/pull/3282


Reviewed-By: Fedor Indutny <fedor.indutny@gmail.com>