Make the socketthread quit before waiting

Occasionally, mythfilldatabase gets hung up on the wait() call being called
via the atexit() hook.  We are trying to exit, so rather than wait for an
uncooperative thread, tell it to quit first.

Related to #7714

mythtv/libs/libmythbase/mythsocketthread.cpp

@@ -58,7 +58,10 @@ void MythSocketThread::ShutdownReadyReadThread(void)
 
     WakeReadyReadThread();
 
-    wait(); // waits for thread to exit
+    if (isRunning()) {
+        quit(); // make the thread exit
+        wait(); // waits for thread to exit
+    }
 
     CloseReadyReadPipe();
 }