A pack of nits in merkleblock.{h|cpp}

Summary: Reformat comment, use early bail and C++11 loops.

Test Plan: make check

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: jasonbcox, teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1436