strcompose: NULL must always be cast when passed to a variadic function

strcompose: NULL must always be cast when passed to a variadic function

The C standard allows NULL to be defined as a bare "0", which will
be passed to variadic functions as an int.  If the function expects
a pointer type, demons fly out of your nose.  strcompose() is such
a function, so make sure that all of its callers cast NULL appropriately.
(None of them did.)  This may be an opportune time to change all of
the callers to spell it opr_strcompose() as well, and avoid using a
reserved identifier, but this change does not do so.

Change-Id: Ia9007a48102da4d0a85a48b41a44e83102304b49
Reviewed-on: http://gerrit.openafs.org/7805
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>