virDomainFormatSchedDef: Avoid false positive NULL dereference

virDomainFormatSchedDef: Avoid false positive NULL dereference

Okay, I admit that our code here is complex. It's not easy to
spot that NULL deref can't really happen here. So it's no wonder
that a dumb compiler fails to see all the connections and
produces the following errors:

  CC       conf/libvirt_conf_la-domain_conf.lo
conf/domain_conf.c: In function 'virDomainDefFormatInternal':
conf/domain_conf.c:22162:22: error: potential null pointer dereference [-Werror=null-dereference]
             if (sched->policy == i)
                 ~~~~~^~~~~~~~
<snip/>
cc1: all warnings being treated as errors

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>