UV: return from read callback if nr is 0

While we usually shouldn't be getting this, it may be possible - in any
event this is UV's way of telling us that it got an EAGAIN and the
buffer we allocated is no longer needed.

Change-Id: Ie99f16b593af203e4eb0a01b4129d64a56429412
Reviewed-on: http://review.couchbase.org/37335
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Brett Lawson <brett19@gmail.com>