Dynamic fee algorithm doesn't check for zero fee

## Summary: Dynamic fee algorithm `Blockchain::get_dynamic_base_fee` calculates the minimal fee per byte from current median block weight and block reward. The comment in the code says `// min_fee_per_byte = round_up( 0.95 * block_reward * ref_weight / (fee_median^2) )`, so it's supposed to round up the result of the division and never return 0 because the argument of `round_up` is always > 0. But the actual code rounds down when doing divisions and can return `min_fee_per_byte = 0`. ## Releases Affected: All current versions of Monero (v0.18.x.x) ## Steps To Reproduce: An attacker could spam the network with transactions until median block weight reaches 42426407 or bigger, at which point `Blockchain::get_dynamic_base_fee` will return 0, allowing 0-fee transactions to be included in mempool and mined. After that, the transaction flood attack will have 0 cost and can continue indefinitely. ## Possible solution `Blockchain::get_dynamic_base_fee` can check for 0 and return `min_fee_per_byte = 1` to avoid this problem. Since it's not a part of consensus, it can be changed in a point release. ## Impact An attacker can eventually flood XMR network with transactions essentially for free, resulting in unlimited blockchain growth.