gcs stream: fix for setting stream rate 0 (equivelant to off)
gcs stream: fix for setting stream rate 0 (equivelant to off)

* untrusted user input here set off an assertion at runtime.
  good thing we caught it!

GaloisInc/ivory-tower-stm32,GaloisInc/ivory-tower-stm32,GaloisInc/ivory-tower-stm32