fix 'attempted to subtract with overflow' for State::inactives (#1668)

Author: jakobhellermann

crates/bevy_ecs/src/schedule/state.rs

@@ -378,7 +378,7 @@ where
     }
 
     pub fn inactives(&self) -> &[T] {
-        &self.stack[0..self.stack.len() - 2]
+        self.stack.split_last().map(|(_, rest)| rest).unwrap()
     }
 }