fix crash in kqueue+fsevents mixed watcher when removing top-level directory
fix crash in kqueue+fsevents mixed watcher when removing top-level directory

Summary:
unordered_map::erase invalidates the current iterator, preventing it
from correctly advancing to the next element.

Reviewed By: xavierd

Differential Revision: D40479399

fbshipit-source-id: 3db650f7db8b4744f68a814e4c5c4d2cb90e1bfb

facebook/watchman,facebook/watchman,facebook/watchman,facebook/watchman,facebook/watchman,facebook/watchman,facebook/watchman,facebook/watchman,facebook/watchman