Fixed using unitialized variables in mouse button handling

See also: https://github.com/minetest/minetest/pull/1707

event.MouseInput.Event is not initialized if event.EventType != EET_MOUSE_INPUT_EVENT and the previous code didn't check that EET_MOUSE_INPUT_EVENT was indeed the event type before checking the MouseEvent (and it was therefore invalid/not initialized).
Since `event.EventType == EET_MOUSE_INPUT_EVENT` is checked in both sides of the "or" you can move it out and do:
```diff
-	if((event.EventType==EET_MOUSE_INPUT_EVENT &&
-			event.MouseInput.Event != EMIE_MOUSE_MOVED) ||
-			(event.MouseInput.Event == EMIE_MOUSE_MOVED &&
-			event.MouseInput.isRightPressed() && getItemAtPos(m_pointer).i != getItemAtPos(m_old_pointer).i)){
+	if(event.EventType == EET_MOUSE_INPUT_EVENT &&
+			(event.MouseInput.Event != EMIE_MOUSE_MOVED ||
+			 (event.MouseInput.Event == EMIE_MOUSE_MOVED &&
+			  event.MouseInput.isRightPressed() &&
+			  getItemAtPos(m_pointer).i != getItemAtPos(m_old_pointer).i)) {
```