Fix build breaker with ATTRIBUTE_NONNULL defs

Fix build breaker with ATTRIBUTE_NONNULL defs

Using "./autogen.sh --system lv_cv_static_analysis=yes" for my daily
Coverity builds resulted in the following error when building:

In file included from util/vircgrouppriv.h:32:0,
                 from util/vircgroup.c:44:
util/vircgroup.h:59:5: error: nonnull argument with out-of-range operand number (argument 1, operand 5)
util/vircgroup.h:74:5: error: nonnull argument references non-pointer operand (argument 1, operand 4)
make[3]: *** [libvirt_util_la-vircgroup.lo] Error 1
make[3]: Leaving directory `/home/jferlan/libvirt.cov.curr/src'
make[2]: *** [all] Error 2
make[2]: Leaving directory `/home/jferlan/libvirt.cov.curr/src'
make[1]: *** [all-recursive] Error 1
make[1]: Leaving directory `/home/jferlan/libvirt.cov.curr'
make: *** [all] Error 2