IOCs: Crash when gracefully quit

As a developer I have noticed that an IOC can segfault when an IOC is closed by `logClientDestroy`, which is called during a graceful shutdown of the IOC (not simply killing the process). This occured when there were many archive monitors on an IOC running in the IOC test framework.

I produced this problem by running the tests for [this version](https://github.com/ISISComputingGroup/IBEX/issues/3679) of the fermi chopper, while I had an archive on two or three of the status MBBIDirect fields. When the IOC tests finished, the IOC crashed.

The debugging pointed to line 337 of [logClient.c](https://github.com/ISISComputingGroup/EPICS-base/blob/712eee734f4e11a8ace680b505bd98ff5ba07c29/src/libCom/log/logClient.c). The error message suggested that the values of pClient were invalid when this line was excecuted. `pClient` was not set to `null`. Could this be because pClient is being changed by the blocks during the if statement?