fix: unsafe usage of unwrap in supply

[gregdhill]

As reported by SRLabs:

[High] Unsafe usage of unwrap() in supply::begin_block

Summary

The conversion of total_supply to FixedPointNumber exceeds accuracy and returns None, which crashes the blockchain once unwrap() is called.

Issue details

The begin_block function from pallet supply will increase the Currency total_issuance based on the inflation rate, using deposit_creating.

Because total_supply can become too large, checked_from_integer will return None and the a panic is being raised because of the unwrap().

We tested the possibility of the crash by using the following values for the genesis of pallet supply:

supply::GenesisConfig::<Runtime> {
    initial_supply: 10_000_000,
    start_height: 100,
    inflation: UnsignedFixedPoint::checked_from_rational(2, 100).unwrap(), // 2%
}

By calling any extrinsic after 100 blocks (the value of start_height), the unwrap() call might fail, potentially causing nodes to panic.

Risk

Once the blockchain reaches the start_height block, if the inflation rate is too high, the chain will crash.

Mitigation

Review the logic of begin_block and replace the checked_from_integer function with an arithmetic safe function.