errlog causes crash on IOC exit

I ran into a case were errlogPrintf() is getting called after the errlog exit handler has run. This handler frees resources and sets a flag (AtExit) when this happens, but errlogPrintf() is ignoring the flag. This results in an attempt to lock a mutex which has been destroyed. Since it is using epicsMutexMustLock() a failure to lock results in an epicsAssert() failure, which calls errlogPrintf()... Which recurses until it overflows the stack.

Solution is to return before the call to msgbufGetFree(). Patch attached.