unit_carrier_drones.lua: fix case of crash when drone was destroyed in same frame as when nanoframe is created.
& fix case where carrier suddenly pause spawning drone until next reload while 1 drone was destroyed while being built

Crash happen because UpdateCoroutine() accidentally skipped 1 frame when 2nd drone was created. This skipped frame give some chance for NIL to pass.
Can happen by burning Armcarry while it spawn 2 drone in succession.