TNT: Fix multiple explosions erasing drops

Any second explosion near a first TNT explosion will punch all
entities found nearby, including item drops. This causes the
item pickup code to think the item was picked up, but by
a `nil` player, thus removing the item.

We query for the immortal entity group, and if the item is in
the immortal group, do not punch the item.

init.lua

@@ -160,12 +160,14 @@ local function entity_physics(pos, radius)
 			local obj_vel = obj:getvelocity()
 			obj:setvelocity(calc_velocity(pos, obj_pos,
 					obj_vel, radius * 10))
+			if not obj:get_armor_groups().immortal then
 				obj:punch(obj, 1.0, {
 					full_punch_interval = 1.0,
 					damage_groups = {fleshy = damage},
 				}, nil)
 			end
 		end
+	end
 end
 
 local function add_effects(pos, radius, drops)