Handle changing target more cleanly

Disconnect so we get no more callbacks and fix potential assert when
changing targets when both are not nullptr.

src/volumemonitor.cpp

@@ -38,10 +38,7 @@ VolumeMonitor::VolumeMonitor(QObject* parent)
 
 VolumeMonitor::~VolumeMonitor()
 {
-    if (m_stream) {
-        pa_stream_unref(m_stream);
-    }
-
+    setTarget(nullptr);
     Context::instance()->unref();
 }
 
@@ -66,14 +63,21 @@ void QPulseAudio::VolumeMonitor::setTarget(QPulseAudio::VolumeObject* target)
     if (target == m_target) {
         return;
     }
-    m_target = target;
-    if (target) {
-        createStream();
-    } else if (m_stream) {
+
+    if (m_stream) {
+        pa_stream_disconnect(m_stream);
         pa_stream_unref(m_stream);
+        pa_stream_set_read_callback(m_stream, nullptr, nullptr);
+        pa_stream_set_suspended_callback(m_stream, nullptr, nullptr);
         m_stream = nullptr;
         Q_EMIT availableChanged();
     }
+
+    m_target = target;
+
+    if (target) {
+        createStream();
+    }
 }
 
 void VolumeMonitor::createStream()