{input,mixer}/alsa: cancel the DeferredMonitor in the destructor

Yet another potential crash bug fix.

src/input/plugins/AlsaInputPlugin.cxx

@@ -100,6 +100,7 @@ public:
 	~AlsaInputStream() {
 		BlockingCall(MultiSocketMonitor::GetEventLoop(), [this](){
 				MultiSocketMonitor::Reset();
+				DeferredMonitor::Cancel();
 			});
 
 		snd_pcm_close(capture_handle);

src/mixer/plugins/AlsaMixerPlugin.cxx

@@ -57,6 +57,7 @@ public:
 	~AlsaMixerMonitor() {
 		BlockingCall(MultiSocketMonitor::GetEventLoop(), [this](){
 				MultiSocketMonitor::Reset();
+				DeferredMonitor::Cancel();
 			});
 	}