Backport dehacked weapon drop crash fix
See https://zandronum.com/tracker/view.php?id=3976, which is caused by an incorrect cast from ADehackedPickup
to AWeapon
leading to an invalid write, resulting in a crash somewhere down the line.
This backports the upstream fix for it: https://github.com/ZDoom/gzdoom/commit/b5d0c5c357e3d74c1b90b9492cf176f14b1c7f72
Edited by Sean Baggaley