Check slots are valid while moving

In some situations (mostly involving mods) slots can be visible to
invtweaks but unusable. While the ModCompatiblity layer should avoid
registering these slots, mods can be volatile so this is an extra layer
of protection to avoid infinite loops.

(This is specifically prompted by a report that forestry backpacks cause errors when InvTweaks attempts to move items into the inventory slot the backpack occupies, as Forestry replaces that slot -- which can be ANY slot in the main inventory -- with a fake slot that does not accept items.)