iovcursor: Set contigptr to NULL if we can't provide a buffer

While technically the status code should be checked for this, it's more
idiomatic to simply see if the pointer has been set to NULL.

Change-Id: I9001bfdbfefab4a1603d57b54481f04ab1462843
Reviewed-on: http://review.couchbase.org/38289
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Brett Lawson <brett19@gmail.com>