Fix a "set but not used" compiler warning in a test

Add a missing assertion whose absence made the test ineffective.
Make the test a little stricter too.

BUG=none
TEST="make check" to run the test + use "-Wall -Werror"

Review URL: https://breakpad.appspot.com/592003

git-svn-id: http://google-breakpad.googlecode.com/svn/trunk@1180 4c0a9323-5329-0410-9bdc-e9ce6186880e