fix off-by-one for wide string size on windows
fix off-by-one for wide string size on windows

Summary:
This would cause us to build a string with a garbage trailing
NUL wide byte and then later fail to match or open files or dirs. :-(

Reviewed By: sid0

Differential Revision: D4332123

fbshipit-source-id: ace6616031c1642188f134bf64dab6e7921756e9

kwlzn/watchman,wez/watchman,nodakai/watchman,kwlzn/watchman,nodakai/watchman,facebook/watchman,kwlzn/watchman,kwlzn/watchman,facebook/watchman,facebook/watchman,facebook/watchman,wez/watchman,wez/watchman,wez/watchman,kwlzn/watchman,nodakai/watchman,wez/watchman,nodakai/watchman,wez/watchman,wez/watchman,nodakai/watchman,facebook/watchman,wez/watchman,nodakai/watchman,kwlzn/watchman,facebook/watchman,nodakai/watchman,facebook/watchman,facebook/watchman,wez/watchman,facebook/watchman,nodakai/watchman,nodakai/watchman,kwlzn/watchman