Checks in post-reviews.py to make more user-friendly.

This is a small fix but wanted most by me, whenever working on master
branch directly with small patches, rather than a dedicate branch,
post-reviews.py doesn't handle this situation but just fail with the
trace back like:

% ./support/post-reviews.py
Running post-review across all of ...
Traceback (most recent call last):
  File "./support/post-reviews.py", line 104, in <module>
    sha = line.split()[0]
IndexError: list index out of range

This is not quite user friendly or make a lot of sense. This patch will
handle the above situation and notice user useful information.

Review: https://reviews.apache.org/r/20364
Fixed lock issue in src/common/lock.cpp.

Review: https://reviews.apache.org/r/20404
Updated CHANGELOG for 0.18.1-rc1.