Avoid incomplete function type in casts

Avoid incomplete function type in casts

clang complains that these casts contain an incomplete function type
(since the function argument is omitted rather than declared to be
void).  Since we just need the cast to pointer type, let the compiler
do it implicitly and pass stock NULL, rather than trying to force a
cast to function-pointer type.

Reviewed-on: https://gerrit.openafs.org/13473
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Cheyenne Wills <cwills@sinenomine.net>
Reviewed-by: Michael Meffie <mmeffie@sinenomine.net>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
(cherry picked from commit 96c0b88947c7aab605170bdca633d3716051a58e)

Change-Id: I950ff8de925a1ca03e50ad7ec394123445b5ce4a
Reviewed-on: https://gerrit.openafs.org/13742
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Marcio Brito Barbosa <mbarbosa@sinenomine.net>
Reviewed-by: Michael Meffie <mmeffie@sinenomine.net>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>