Fix segfault on a mesh with convex inertia that no geom references

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

src/user/user_model.cc

@@ -5279,12 +5279,19 @@ void mjCModel::TryCompile(mjModel*& m, mjData*& d, const mjVFS* vfs) {
     if (geoms_[i]->mesh &&
         (geoms_[i]->spec.type == mjGEOM_MESH ||
          geoms_[i]->spec.type == mjGEOM_SDF) &&
-        (geoms_[i]->spec.contype || geoms_[i]->spec.conaffinity || is_in_pair ||
-         geoms_[i]->mesh->spec.inertia == mjMESH_INERTIA_CONVEX)) {
+        (geoms_[i]->spec.contype || geoms_[i]->spec.conaffinity || is_in_pair)) {
       geoms_[i]->mesh->SetNeedHull(true);
     }
   }
 
+  // convex inertia is computed from the hull, so it is needed whether or not
+  // any geom references the mesh
+  for (mjCMesh* mesh : meshes_) {
+    if (mesh->spec.inertia == mjMESH_INERTIA_CONVEX) {
+      mesh->SetNeedHull(true);
+    }
+  }
+
   // automatically set nuser fields
   SetNuser();