mixer/alsa: reset the MultiSocketMonitor in the destructor

Fixes potential crash bug.

NEWS

@@ -3,6 +3,8 @@ ver 0.20.5 (not yet released)
   - id3: fix memory leak on corrupt ID3 tags
 * decoder
   - sidplay: don't require libsidutils when building with libsidplayfp
+* mixer
+  - alsa: fix crash bug
 
 ver 0.20.4 (2017/02/01)
 * input

src/mixer/plugins/AlsaMixerPlugin.cxx

@@ -23,6 +23,7 @@
 #include "output/OutputAPI.hxx"
 #include "event/MultiSocketMonitor.hxx"
 #include "event/DeferredMonitor.hxx"
+#include "event/Call.hxx"
 #include "util/ASCII.hxx"
 #include "util/ReusableArray.hxx"
 #include "util/Domain.hxx"
@@ -53,6 +54,12 @@ public:
 		DeferredMonitor::Schedule();
 	}
 
+	~AlsaMixerMonitor() {
+		BlockingCall(MultiSocketMonitor::GetEventLoop(), [this](){
+				MultiSocketMonitor::Reset();
+			});
+	}
+
 private:
 	virtual void RunDeferred() override {
 		InvalidateSockets();