Ensure a2j is started before attempting to stop it

When stopping jackd, ensure that a2j is actually started before
attempting to stop it. Otherwise we get an error and jackd is not
stopped.

Signed-off-by: Alex Tsitsimpis <alex.tsitsimpis@gmail.com>

src/cadence.py

@@ -1667,7 +1667,7 @@ def slot_JackServerStart(self):
 
     @pyqtSlot()
     def slot_JackServerStop(self):
-        if gDBus.a2j:
+        if gDBus.a2j and bool(gDBus.a2j.is_started()):
             gDBus.a2j.stop()
         try:
             gDBus.jack.StopServer()