Deobfuscate cleanup path in clnt_bck_create(..)

Similar to r300836, cl and ct will always be non-NULL as they're allocated
using the mem_alloc routines, which always use malloc(..., M_WAITOK).

Deobfuscating the cleanup path fixes a leak where if cl was NULL and
ct was not, ct would not be free'd, and also removes a duplicate test for
cl not being NULL.

Approved by: re (gjb)
Differential Revision: https://reviews.freebsd.org/D6801
MFC after: 1 week
Reported by: Coverity
CID: 1229999
Reviewed by: cem
Sponsored by: EMC / Isilon Storage Division