Issue Description:

It is possible to put a backpack with the right upgrades into itself.

What happens:

The results vary - I've had both "backpack vanishes when closed, never to be seen again without pulling it out of a backup" and "the server tries to parse the recursing backpack, buffer overflows, and crashes entirely."

What you expected to happen:

It shouldn't be possible in the first place, but if somehow some weird mod interaction makes it occur it should be able to fail gracefully, ideally ejecting the backpack into the world.

Steps to reproduce (important):

1. Make any backpack.
2. Apply the Advanced Nesting Upgrade to it.
3. Put the backpack in the hotbar (let's say in slot 3.)
4. Open the backpack. Hover over an entry in the inventory.
5. Using the number key associated with the hotbar slot the backpack is in (in this case 3), swap the backpack and the item in the inventory.
6. Close the backpack.

Affected Versions (Do not use "latest"):

• IronBackpacks: 2.2.23
• Minecraft: 1.10.2
• Forge: 12.8.3.2254
• Other Conflicting Mods (may be irrelevant to your issue):

I swear this was fixed a long time ago...

Looks like something similar, with the number keys causing issues, was fixed via Issue #18 (a long time ago), but it has apparently popped back up.