On Initialize should panic with input 0

[shawntabrizi]

I am fixing benchmarks in pallet_bounties because the benchmarks took advantage of some behavior when on_initialize(0) was executed.

However, in a real blockchain, there will never be an on_initialize(0), so this was a hack which later broke when it shouldn't have.

I think within the macros, we should implement some sort of debug panic (or real panic) if the input to on_initialize is zero, to discourage anyone from running the function in this scenario.

[shawntabrizi]

The same should be applied to any of the pallet hooks where block number comes in. on_finalize, etc...