protocol/validation: check for int64 overflow when verifying tx amounts
protocol/validation: check for int64 overflow when verifying tx amounts

Closes chain/chainprv#1621
Reviewers: da39a3ee5e6b4b0d3255bfef95601890afd80709@bobg

chain/chain,chain/chain,chain/chain,chain/chain,chain/chain,chain/chain