Fix possible invalid object reference in TNT mod

mods/tnt/init.lua

@@ -159,6 +159,7 @@ local function entity_physics(pos, radius, drops)
 	local objs = minetest.get_objects_inside_radius(pos, radius)
 	for _, obj in pairs(objs) do
 		local obj_pos = obj:get_pos()
+		if obj_pos then
 		local dist = math.max(1, vector.distance(pos, obj_pos))
 
 		local damage = (4 / dist) * radius
@@ -200,6 +201,7 @@ local function entity_physics(pos, radius, drops)
 				end
 			end
 		end
+		end
 	end
 end