Added IsValid check for physics object

To prevent errors like this:
[ERROR] gamemodes/darkrp/entities/entities/spawned_shipment/init.lua:35: Tried to use invalid object (type IPhysicsObject) (Object was NULL or not of the right type)
Merge pull request #1680 from MStruntze/patch-1

Added IsValid check for physics object
Returning in "PlayerPickupDarkRPWeapon" does nothing fix
Added info about fix in hookStub
Fixed bug introduced in #1683
Majorly refactored door ram code
Added onDoorRamUsed hook.

Fixes #1678
Merge branch 'master' into workshop