unix,stream: fix getting the correct fd for a handle

On OSX it's possible that the fd is replaced, so use the proper libuv
API to get the correct fd.

PR-URL: nodejs#6753
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

src/stream_wrap.cc

@@ -91,7 +91,7 @@ int StreamWrap::GetFD() {
   int fd = -1;
 #if !defined(_WIN32)
   if (stream() != nullptr)
-    fd = stream()->io_watcher.fd;
+    uv_fileno(reinterpret_cast<uv_handle_t*>(stream()), &fd);
 #endif
   return fd;
 }