event/SignalMonitor: use BoundMethod instead of raw function pointer

src/event/SignalMonitor.cxx

@@ -158,7 +158,7 @@ SignalMonitorFinish()
 	sa.sa_handler = SIG_DFL;
 
 	for (unsigned i = 0; i < MAX_SIGNAL; ++i) {
-		if (signal_handlers[i] != nullptr) {
+		if (signal_handlers[i]) {
 			x_sigaction(i, sa);
 			signal_handlers[i] = nullptr;
 		}
@@ -179,7 +179,7 @@ SignalMonitorGetEventLoop()
 void
 SignalMonitorRegister(int signo, SignalHandler handler)
 {
-	assert(signal_handlers[signo] == nullptr);
+	assert(!signal_handlers[signo]);
 #ifndef USE_SIGNALFD
 	assert(!signal_pending[signo]);
 #endif
@@ -209,7 +209,7 @@ SignalMonitor::OnSocketReady(unsigned)
 	int signo;
 	while ((signo = fd.Read()) >= 0) {
 		assert(unsigned(signo) < MAX_SIGNAL);
-		assert(signal_handlers[signo] != nullptr);
+		assert(signal_handlers[signo]);
 
 		signal_handlers[signo]();
 	}

src/event/SignalMonitor.hxx

@@ -26,7 +26,9 @@ class EventLoop;
 
 #ifndef WIN32
 
-typedef void (*SignalHandler)();
+#include "util/BindMethod.hxx"
+
+typedef BoundMethod<void()> SignalHandler;
 
 /**
  * Initialise the signal monitor subsystem.