CoreAudio: Ensure devices are restarted correctly after changing sample rate

f1b6bbc921 erroneously replaced

    callback = newCallback;
    previousCallback = callback;

with

    previousCallback = std::exchange (callback, newCallback);

It looks like previousCallback is intended to hold the last active
callback, so the previous version was correct after all.

modules/juce_audio_devices/native/juce_mac_CoreAudio.cpp

@@ -1555,7 +1555,7 @@ public:
                 newCallback->audioDeviceAboutToStart (this);
 
             const ScopedLock sl (callbackLock);
-            previousCallback = std::exchange (callback, newCallback);
+            previousCallback = callback = newCallback;
         }
     }