One more C++11 loop in validation.cpp
One more C++11 loop in validation.cpp

Summary: As per title.

Test Plan:
  make check

Reviewers: #bitcoin_abc, schancel

Reviewed By: #bitcoin_abc, schancel

Subscribers: jasonbcox, teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1468

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