feat(contracts-eth): don't allow zero-address initialization in AdminControlled

[ashutoshvarma]

Fix #844

Note: -

Instead of adding a new require(_admin != address(0) (and copying the comment to describe why) I used existing verifyAdminAddress() to reduce redundancy but the existing method name and require error message might not be appropriate. Also, the fact that newAdmin != admin will hit first rather than zero address check.