ch4/posix: fix post_init when local_size is 1

Pull Request Description

When local size is 1, comm_world->node_comm does not exist and the Allgather result in segfaults.

This fixes the segfaults when setting MPIR_CVAR_CH4_SHM_POSIX_TOPO_ENABLE=1 and node local size is 1.o

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