Seg fault on skin element?

I don't know if this is from `dm_control` or `MuJoCo`, but rendering with glfw on mac gives my segfault when I try to use ``.

Sample script

```
from dm_control import mjcf
from dm_control import composer
from dm_control import viewer
from dm_control import suite

width = 320
height = 240
mjcf_str = """

"""
world = mjcf.from_xml_string(mjcf_str)
world_entity = composer.ModelWrapperEntity(world)
task = composer.NullTask(world_entity)
task.control_timestep = 0.02
env = composer.Environment(task)
viewer.launch(env)
```