bigalloc: set seg->nalloc after realloc

(yo dawg, i hurd u like alloc ...)

Change-Id: Ibb9369680ab1e13a2477901a323cedc68b4cfbc7
Reviewed-on: http://review.couchbase.org/36279
Reviewed-by: Brett Lawson <brett19@gmail.com>
Tested-by: Mark Nunberg <mnunberg@haskalah.org>