Seg Fault in CombinerGenerator

[miaoyinb]

Bug Description

Using CombinerGenerator in distrubuted_mesh mode with very small mesh leads to seg fault.

Steps to Reproduce

[Mesh]
  [gmg_0]
    type = GeneratedMeshGenerator
    dim = 2
    nx = 1
    subdomain_ids = 1
  []
  [gmg_1]
    type = GeneratedMeshGenerator
    dim = 2
    nx = 4
    subdomain_ids = 4
  []
  [cmbn]
      type = CombinerGenerator
      inputs = 'gmg_0 gmg_1'
      positions = '0 2 0
                   2 0 0'
  []
[]

Running this in distributed_mesh mode (mpirun -n 2 moose_test-opt -i single_scalar_alt.i --mesh-only --distributed-mesh) leads to a seg fault. It seems to be related to that the mesh is too small. So far this issue seems to only happen when one input mesh has only one element and the other mesh is bigger.

Impact

Prevent such use of distributed_mesh mode.

[Optional] Diagnostics

No response

[miaoyinb]

@GiudGiud I think the issue may originate from the libMesh::UnstructuredMesh::copy_nodes_and_elements().

However, if we sort the input meshes so that we start from the mesh with most elements first, the issue seems to be averted. If you think this is a okay solution for now, I can do a PR.