Remove spurious NULL checks

Remove spurious NULL checks

clang 3.5 is more aggressive about these checks than the previous
FreeBSD system compiler, so new warnings (which became errors)
appeared on FreeBSD 11-CURRENT.

In afs_dcache.c, checking &tdc->f for NULL-ness has no effect.
The struct fcache f member of struct dcache is an ordinary structure
element; its address will be the value of tdc plus the offset of
f within struct dcache, which will not be NULL even if tdc is NULL.

In ubik_db_if.c, udbHandle is a file-scope global and thus has
allocated storage; the address of a member variable will never
be NULL.  The 0 it was compared against was spelled RX_SECIDX_NULL,
which shows the intended check, which is for the value of the
uh_scIndex member variable, not its address.

In afscp_server.c, srv->conns can never be NULL since conns is a member
variable of struct afscp_server (of array type, containing pointers
to struct rx_connection).  Comparing the array member variable against
NULL is comparing the address of the array, which is never NULL since
it is not allocated separately from struct afscp_server.

In fssync-debug.c, state.vop->partName is never NULL because
common_volop_prolog always allocates for state.vop, and the
partName member variable of struct fssync_state is of array type,
and thus is not separately allocated from the containing structure.

Change-Id: I03e1332d8a3320f1a4d303b444985648a207116e
Reviewed-on: http://gerrit.openafs.org/11739
Reviewed-by: Chas Williams - CONTRACTOR <chas@cmf.nrl.navy.mil>
Reviewed-by: Perry Ruiter <pruiter@sinenomine.net>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Tested-by: BuildBot <buildbot@rampaginggeek.com>