libCom/errlog: Clean up possible problem at shutdown.

Don't use epicsMutexMustLock() in msgbufGetFree().
When we're shutting down, the pvtData.msgQueueLock gets destroyed.
This makes msgbufGetFree() return NULL if the lock is dead.
The epicsMutexMustLock() routine asserts(), which recurses...

src/libCom/error/errlog.c

@@ -552,7 +552,9 @@ static char *msgbufGetFree(int noConsoleMessage)
 {
     msgNode *pnextSend;
 
-    epicsMutexMustLock(pvtData.msgQueueLock);
+    if (epicsMutexLock(pvtData.msgQueueLock) != epicsMutexLockOK)
+        return 0;
+
     if ((ellCount(&pvtData.msgQueue) == 0) && pvtData.missedMessages) {
         int nchar;