UPstream::shutdown misbehaves with external initialisation

When MPI has been initialised without OpenFOAM itself being a parallel job, the UPstream::shutdown causes a segfault.

cross-ref: precice issue 293

changed milestone to %v2306

added bug label

assigned to @mark

Needs an upper bounds check on the communicator index since MPI may have been initialized outside of OpenFOAM, in which case there are no Pstream communicator equivalents

mentioned in commit 3de090e6

mentioned in commit 20c7f097

mentioned in commit 68bed4f1

mentioned in commit 50b44dbf

mentioned in commit 8f214f74

mentioned in commit d189f95c

mentioned in commit 00ebd4c2

mentioned in commit b1d62979

closed