Fix memory leak and possibly use-after-free in user_mesh.cc.
Fix memory leak and possibly use-after-free in user_mesh.cc.

The memory handling in the asset loading logic in user_mesh.cc is currently flipped: it frees the asset content buffer when reading from VFS but not from the filesystem. This causes the VFS object to leave these functions in an invalid state, while leaking locally allocated buffers that hold contents read from the filesystem.

PiperOrigin-RevId: 466643390
Change-Id: I162221e019861f23fb61308facd63dbb0422e113

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