Don't misidentify RTs as UAVs if UAVStartSlot is mis-set to 0
Protect against the case of setting UAVStartSlot=0, NumUAVs=0

* Caused the render state tracking to get confused (as UAVStartSlot is
  used to determine the number of RTVs) and led to weird inconsistent
  states being restored. Most obviously led to RTs not being set when
  they should have been on replaying and drawcalls going missing.