st/mesa: do not take util_logbase2 of a negative size

It's possible to get all width, height and depth as negative here, and
if we take the util_logbase2 of that, we get a nonsense value that might
be outside of the range the driver supports. So let's clamp to zero here
to avoid having to make the drivers overly pessimistic about what the
state-tracker demands.

Reviewed-by: Adam Jackson <ajax@redhat.com>
Reviewed-by: Dave Airlie <airlied@redhat.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/10689>

src/mesa/state_tracker/st_cb_texture.c

@@ -3162,7 +3162,7 @@ st_TestProxyTexImage(struct gl_context *ctx, GLenum target,
       }
       else {
          /* assume a full set of mipmaps */
-         pt.last_level = util_logbase2(MAX3(width, height, depth));
+         pt.last_level = util_logbase2(MAX4(width, height, depth, 0));
       }
 
       return st->screen->can_create_resource(st->screen, &pt);