restore inadvertently deleted code

This has an interesting history.  In the original Volition source release, global damage was set to originate from the shockwave position, which would have avoided the fireball issue fixed by scp-fs2open#6882.  (Technically, though, this was only done for huge ships which would have still left the problem for smaller ships.)  In c2716d4, this code was changed to run in non-HTL mode in an attempt to fix a bug.  Then in 70529db, as part of the removal of non-HTL mode, the code was entirely deleted.  So, this restores the code.

The scp-fs2open#6882 fix is still valid, both because it works for non-huge ships, and because shockwaves may not necessarily originate from the subobject being destroyed.  The scp-fs2open#6882 fix also subsumes the effect of the restored code, but it's still good to restore the code in case future development may depend on it.

Author: Goober5000

code/ship/shiphit.cpp

@@ -3080,6 +3080,11 @@ void ship_apply_global_damage(object *ship_objp, object *other_obj, const vec3d
 		// shield_quad = quadrant facing the force_center
 		shield_quad = get_quadrant(&local_hitpos, ship_objp);
 
+		// world_hitpos use force_center for shockwave
+		// Goober5000 check for NULL
+		if (other_obj && (other_obj->type == OBJ_SHOCKWAVE) && (Ship_info[Ships[ship_objp->instance].ship_info_index].is_huge_ship()))
+			world_hitpos = *force_center;
+
 		int wip_index = -1;
 		if(other_obj != nullptr && other_obj->type == OBJ_SHOCKWAVE)
 			wip_index = shockwave_get_weapon_index(other_obj->instance);