vms: Fix deadlock when stopping timers during spdagvm.VM.Shutdown()

refs #66

vms/spdagvm/vm.go

@@ -134,7 +134,11 @@ func (vm *VM) Shutdown() {
 		return
 	}
 
+	// There is a potential deadlock if the timer is about to execute a timeout.
+	// So, the lock must be released before stopping the timer.
+	vm.ctx.Lock.Unlock()
 	vm.timer.Stop()
+	vm.ctx.Lock.Lock()
 	if err := vm.baseDB.Close(); err != nil {
 		vm.ctx.Log.Error("Closing the database failed with %s", err)
 	}