qemu: cgroup: Fix crash if starting nographics guest

We can dereference graphics[0] even if guest has no graphics device
configured. I screwed this up in a216e648

https://bugzilla.redhat.com/show_bug.cgi?id=1014088
(cherry picked from commit a924d9d0)

src/qemu/qemu_cgroup.c

@@ -490,9 +490,10 @@ qemuSetupDevicesCgroup(virQEMUDriverPtr driver,
 
     if (vm->def->nsounds &&
         ((!vm->def->ngraphics && cfg->nogfxAllowHostAudio) ||
-         ((vm->def->graphics[0]->type == VIR_DOMAIN_GRAPHICS_TYPE_VNC &&
+         (vm->def->graphics &&
+          ((vm->def->graphics[0]->type == VIR_DOMAIN_GRAPHICS_TYPE_VNC &&
            cfg->vncAllowHostAudio) ||
-           (vm->def->graphics[0]->type == VIR_DOMAIN_GRAPHICS_TYPE_SDL)))) {
+           (vm->def->graphics[0]->type == VIR_DOMAIN_GRAPHICS_TYPE_SDL))))) {
         rv = virCgroupAllowDeviceMajor(priv->cgroup, 'c', DEVICE_SND_MAJOR,
                                        VIR_CGROUP_DEVICE_RW);
         virDomainAuditCgroupMajor(vm, priv->cgroup, "allow", DEVICE_SND_MAJOR,