core/account: check bounds of witness on annotate
core/account: check bounds of witness on annotate

Fixes a bug that caused an out of bounds panic.

Closes chain/chainprv#1348.
Reviewers: da39a3ee5e6b4b0d3255bfef95601890afd80709@kr




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