fix: inventory slot bounds check (#900)

## Summary
- prevent panic when mutating invalid inventory slots

## Testing
- `cargo fmt --all -- --check` *(fails: could not download toolchain)*
- `cargo test --locked --all --no-run` *(fails: could not download
toolchain)*

---------

Co-authored-by: TestingPlant <44930139+TestingPlant@users.noreply.github.com>

Author: andrewgazelka

crates/hyperion-inventory/src/lib.rs

@@ -326,9 +326,9 @@ impl Inventory {
         &mut self,
         index: u16,
     ) -> Result<&mut ItemSlot, InventoryAccessError> {
-        let index = usize::from(index);
-        let slot = &mut self.slots[index];
-        Ok(slot)
+        self.slots
+            .get_mut(usize::from(index))
+            .ok_or(InventoryAccessError::InvalidSlot { index })
     }
 
     /// Returns remaining [`ItemStack`] if not all of the item was added to the slot