[bug][modding] display error for rockets with submunitions

I was attempting to make a rocket launcher (“SIM_MissileModule” and has_splines = 1) with submunitions. the problem is that when the rocket splits off into multiple warheads, only one of them still has the black smoke trail. All the others have no trail. Everything else works fine. Probably just an oversight since rockets wouldn’t normally have submunitions.
My reason for having submunitions was to fire multiple rockets from one launcher at the same time: decoy_release_range = max_range. It makes a very cool effect since they curve in different directions after launch.