darwin: fix spurious uv_write2() segfault

We abuse uv_write2() to send over UDP handles to child processes.
Don't call uv__stream_fd() on those handles, it's a macro that on OS X
evaluates to a function that operates on a uv_stream_t with a couple of
OS X specific fields. On other Unices it does (handle)->io_watcher.fd,
which works but only by accident.

Fixes joyent/node#4870.