nwfilter: fix crash when adding non-existing nwfilter

Adding non-existing nwfilter to a network interface device without any
nwfilter specified will crash libvirt daemon with segfault. The reason is
that the nwfilter is not found an libvirt will try to restore old
nwfilter configuration but there is no nwfilter specified.
Signed-off-by: NPavel Hrdina <phrdina@redhat.com>

src/conf/nwfilter_conf.c

@@ -2778,7 +2778,7 @@ virNWFilterObjFindByName(virNWFilterObjListPtr nwfilters, const char *name)
 
     for (i = 0; i < nwfilters->count; i++) {
         virNWFilterObjLock(nwfilters->objs[i]);
-        if (STREQ(nwfilters->objs[i]->def->name, name))
+        if (STREQ_NULLABLE(nwfilters->objs[i]->def->name, name))
             return nwfilters->objs[i];
         virNWFilterObjUnlock(nwfilters->objs[i]);
     }