Don't memset SEvent directly (#15359)

Fixes a compiler warning by manually zeroing the tag and the largest union member instead

irr/include/IEventReceiver.h

@@ -18,7 +18,7 @@ enum EEVENT_TYPE
 	to mouse or keyboard events. When a GUI element receives an event it will either
 	process it and return true, or pass the event to its parent. If an event is not absorbed
 	before it reaches the root element then it will then be passed to the user receiver. */
-	EET_GUI_EVENT = 0,
+	EET_GUI_EVENT = 1,
 
 	//! A mouse input event.
 	/** Mouse events are created by the device and passed to IrrlichtDevice::postEventFromUser
@@ -332,6 +332,9 @@ struct SEvent
 		//! True if ctrl was also pressed
 		bool Control : 1;
 
+		//! Is this a simulated mouse event generated by the engine itself?
+		bool Simulated : 1;
+
 		//! A bitmap of button states. You can use isButtonPressed() to determine
 		//! if a button is pressed or not.
 		u32 ButtonStates;
@@ -347,9 +350,6 @@ struct SEvent
 
 		//! Type of mouse event
 		EMOUSE_INPUT_EVENT Event;
-
-		//! Is this a simulated mouse event generated by Minetest itself?
-		bool Simulated;
 	};
 
 	//! Any kind of keyboard event.
@@ -543,8 +543,9 @@ struct SEvent
 	};
 
 	SEvent() {
-		// would be left uninitialized in many places otherwise
-		MouseInput.Simulated = false;
+		EventType = static_cast<EEVENT_TYPE>(0);
+		// zero the biggest union member we have, which clears all others too
+		memset(&JoystickEvent, 0, sizeof(JoystickEvent));
 	}
 };