Force handle to be NULL on lcb_create() failure

lcb_create() can leave garbage in the pointer even if the call itself
was failed. This behaviour could lead to illegal memory access on GC

Change-Id: Iab23068fe22df4931a31d78b274d1a7782f1950c
Related-To: http://review.couchbase.org/27636
Reviewed-on: http://review.couchbase.org/27639
Tested-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Reviewed-by: Matt Ingenthron <matt@couchbase.com>