CCBC-515: Check that config is not NULL for rget3

This also performs the same check in some other API entry points as well

Change-Id: I792c5d5cb4e930c584c0d5509f590d43b54a9e83
Reviewed-on: http://review.couchbase.org/41464
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>