needhull_ was only set while iterating geoms, so a mesh whose inertia is
computed from its convex hull never got one unless some geom pointed at
it. mjCMesh::ComputeVolume and ComputeInertia then read graph_[1] and
GraphFaces() off a null pointer and the compiler crashed.
Whether a mesh needs its hull for inertia is a property of the mesh, not
of any geom, so move that condition out of the geom loop and apply it to
every mesh.
Fixes#3431
VihaanAgarwal
95f05402f1