CBD-65: Fix asym/3

vtree_split:asym/3 broke when the length of the MBB is zero. The
solution is to return zero immediately.

Change-Id: I443e01ca3c7233eb03fa32c751bf7aad10b80ecb
Reviewed-on: http://review.couchbase.org/21025
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Filipe David Borba Manana <fdmanana@gmail.com>
Reviewed-by: Volker Mische <volker.mische@gmail.com>