nv50: fix segfault when there's gaps in enabled texture units

Tested with progs/demos/multiarb.

src/gallium/drivers/nv50/nv50_state_validate.c

@@ -356,13 +356,16 @@ viewport_uptodate:
 	if (nv50->dirty & NV50_NEW_SAMPLER) {
 		int i;
 
-		so = so_new(nv50->sampler_nr * 8 + 3, 0);
+		so = so_new(nv50->sampler_nr * 9 + 2, 0);
 		so_method(so, tesla, NV50TCL_CB_ADDR, 1);
 		so_data  (so, NV50_CB_TSC);
-		so_method(so, tesla, NV50TCL_CB_DATA(0) | 0x40000000,
-			nv50->sampler_nr * 8);
-		for (i = 0; i < nv50->sampler_nr; i++)
+		for (i = 0; i < nv50->sampler_nr; i++) {
+			if (!nv50->sampler[i])
+				continue;
+
+			so_method(so, tesla, NV50TCL_CB_DATA(0) | (2<<29), 8);
 			so_datap (so, nv50->sampler[i]->tsc, 8);
+		}
 		so_ref(so, &nv50->state.tsc_upload);
 		so_ref(NULL, &so);
 	}

src/gallium/drivers/nv50/nv50_tex.c

@@ -157,6 +157,9 @@ nv50_tex_validate(struct nv50_context *nv50)
 	for (unit = 0; unit < nv50->miptree_nr; unit++) {
 		struct nv50_miptree *mt = nv50->miptree[unit];
 
+		if (!mt)
+			continue;
+
 		so_method(so, tesla, NV50TCL_CB_DATA(0) | 0x40000000, 8);
 		if (nv50_tex_construct(nv50, so, mt, unit)) {
 			NOUVEAU_ERR("failed tex validate\n");