nv50: access only the available amount of textures

The textures array is defined as a number of PIPE_MAX_SAMPLERS per shader stage.
Currently nv50 driver handles only 3 shader stages, thus we wreck chaos when
accessing array-out-of-bounds.

Fixes a segfault in piglit/bin/arb_texture_buffer_object-data-sync -fbo -auto

Cc: 9.1 9.2 10.0 <mesa-stable@lists.freedesktop.org>
Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
Reviewed-by: Ilia Mirkin <imirkin@alum.mit.edu>

src/gallium/drivers/nouveau/nv50/nv50_context.c

@@ -165,7 +165,7 @@ nv50_invalidate_resource_storage(struct nouveau_context *ctx,
    }
 
    if (res->bind & PIPE_BIND_SAMPLER_VIEW) {
-      for (s = 0; s < 5; ++s) {
+      for (s = 0; s < 3; ++s) {
       for (i = 0; i < nv50->num_textures[s]; ++i) {
          if (nv50->textures[s][i] &&
              nv50->textures[s][i]->texture == res) {