Add an instanceof check

[Mrbysco]

Currently if a pack dev allows other mods quivers to go into the quiver curios slot it'll cause a nullpointer when QuiverItem#getInventory is called. The instanceof makes sure it won't get to that point if it isn't a quiver from this mod.

An example of the nullpointer that would be fixed:
https://pastebin.com/0cgXQqF2

[Nyfaria]

Why are other non-nyf's quivers quivers going into my quiver slot?

[Darkosto]

allowing other quivers to go into that slot makes it multi-purpose, and ties multiple mods together in the same modpack. It also adds a level of immersion for players.

[Mrbysco]

There's always the possibility of multiple mods adding a curios slot and use the same identifier. Going off the Curios description identical identifiers will be merged together