Don't overrun the chances array when synching > MAX_SLOTS
On a device with more than 256 slots we would read (and copy) past our changes stack-allocated changes array. Fix this by capping to MAX_SLOTS though this also requires us to memset the target where it is larger than MAX_SLOTS.
There are no real devices with 256+ slots, so this is a theoretical issue only.
Fixes #11 (closed)
cc @mforney