Fix null pointer dereference in mj_loadModel.
Fix null pointer dereference in mj_loadModel.

mj_makeModel returns NULL if any of the array sizes are negative. Callers of the function need to handle that case.

PiperOrigin-RevId: 457948558
Change-Id: I9a5c8c0d3293ed0a92c5791f00d3ed34c848e024

deepmind/mujoco,deepmind/mujoco,deepmind/mujoco,deepmind/mujoco,deepmind/mujoco