Fix NaN check reading wrong index in MPI unpack #4443

Job	Run time
Lint Gate	1m 21s
Detect File Changes	11s
Github (macos, mpi, debug, false)	1h 11m 53s
Github (ubuntu, mpi, no-debug, false)	2h 16m 25s
Github (ubuntu, mpi, no-debug, true)	1h 33m 24s
Github (ubuntu, no-mpi, single, no-debug, false)	30m 32s
Github (ubuntu, mpi, debug, false)	2h 41m 12s
Github (ubuntu, mpi, debug, true)	1h 23m 12s
Georgia Tech \| Phoenix (gpu-acc)	9s
Github (macos, mpi, no-debug, false)	1h 17m 51s
Oak Ridge \| Frontier (cpu)	15h 5m 12s
Oak Ridge \| Frontier (gpu-acc)	15h 5m 12s
Georgia Tech \| Phoenix (gpu-omp)	10s
Oak Ridge \| Frontier (AMD) (gpu-omp)	1s
Oak Ridge \| Frontier (gpu-omp)	1s
Georgia Tech \| Phoenix (cpu)	10s
Oak Ridge \| Frontier (AMD) (cpu)	1s
	1d 17h 6m 57s

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