LibAudio: Disconnect from and unref the PulseAudio context with lock

We don't want to pull the stream out from under our PulseAudio main
loop, so call these with the lock to ensure that nothing is touching
them.

The `pa_threaded_mainloop_stop()` call does not require lock as it sets
a flag to tell the main loop to exit.

Userland/Libraries/LibAudio/PulseAudioWrappers.cpp

@@ -105,8 +105,11 @@ PulseAudioContext::PulseAudioContext(pa_threaded_mainloop* main_loop, pa_mainloo
 
 PulseAudioContext::~PulseAudioContext()
 {
-    pa_context_disconnect(m_context);
-    pa_context_unref(m_context);
+    {
+        auto locker = main_loop_locker();
+        pa_context_disconnect(m_context);
+        pa_context_unref(m_context);
+    }
     pa_threaded_mainloop_stop(m_main_loop);
     pa_threaded_mainloop_free(m_main_loop);
 }