protocol/vm: Fix a bug in opMaxTime
protocol/vm: Fix a bug in opMaxTime

Was failing to interpret a maxtime of 0 as "unlimited."

Closes chain/chainprv#1552.
Reviewers: da39a3ee5e6b4b0d3255bfef95601890afd80709@kr




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