Changes to impact effects do not propegate to dinky explosions in modular tables

If a missile has an impact effect defined but not a dinky impact effect, then the dinky effect will be made to be the same as the regular effect. *But* this is only done if `first_time`, so if a missile is created, with or without an impact effect, and a modular table sets a new impact effect, the dinky effect will not be changed to match.

I have a missile affected by this, and checked my diagnosis by just removing the condition (on line ~1938 of weapons.cpp), but that is obviously not a viable actual solution. Logically, I think that if the dinky and impact explosion were the same before the impact was changed then most likely the dinky should be kept in sync, but I'm not sure that'll always be true.