Fixes: 1. Do not allow min difficulty below 2 and max difficulty above 128 2. Validate that min reserve * reserve units do not overflow
Fixes:
1. Do not allow min difficulty below 2 and max difficulty above 128
2. Validate that min reserve * reserve units do not overflow

fimkrypto/nxt,Ziftr/nxt,fimkrypto/nxt,Ziftr/nxt,fimkrypto/nxt,Ziftr/nxt,fimkrypto/nxt