drm/nouveau/kms/nv50: perform null check on msto[i] rathern than msto

The null check on the array msto is incorrect since msto is never
null. The null check should be instead on msto[i] since this is
being dereferenced in the call to drm_mode_connector_attach_encoder.

Thanks to Emil Velikov for pointing out the mistake in my original
fix and for suggesting the correct fix.

Detected by CoverityScan, CID#1375915 ("Array compared against 0")

Fixes: f479c0ba ("drm/nouveau/kms/nv50: initial support for DP 1.2 multi-stream")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>

drivers/gpu/drm/nouveau/nv50_display.c

@@ -3141,7 +3141,7 @@ nv50_mstc_new(struct nv50_mstm *mstm, struct drm_dp_mst_port *port,
 	mstc->connector.funcs->reset(&mstc->connector);
 	nouveau_conn_attach_properties(&mstc->connector);
 
-	for (i = 0; i < ARRAY_SIZE(mstm->msto) && mstm->msto; i++)
+	for (i = 0; i < ARRAY_SIZE(mstm->msto) && mstm->msto[i]; i++)
 		drm_mode_connector_attach_encoder(&mstc->connector, &mstm->msto[i]->encoder);
 
 	drm_object_attach_property(&mstc->connector.base, dev->mode_config.path_property, 0);