Segfault when a process is assigned no meshes

[JakeOShannessy]

This came up when discussing #9968. When manually assigning meshes to MPI processes, if a process is skipped it produces a segfault.

The input below will produce such a segfault.

&HEAD CHID='process-skip-test' /
&MESH ID='MESH1' IJK=10,10,10, XB=0,1,0,1,0,1 MPI_PROCESS=0 /
&MESH ID='MESH2' IJK=10,10,10, XB=1,2,0,1,0,1 MPI_PROCESS=2 /
&MESH ID='MESH3' IJK=10,10,10, XB=2,3,0,1,0,1 MPI_PROCESS=2 /
&TAIL /

This should still be an error, but it's something that can be trivially caught with an error message produced. #9968 addresses this, but if that's not merged, it's a simple check to implement.

[mcgratta]

I'll fix this.

[mcgratta]

I patched a few holes regarding the use of MPI_PROCESS.

[JakeOShannessy]

Certainly seems to address it. Do you normally close issues on merge with master or with a new release?

[mcgratta]

We close with user confirmation. We can always reopen if I broke something.