Fix a bug in Shadow::IsBeginningOfBlockBody

The beginning of a block might be in its trailer in the case of a 0 bytes block.

BUG=
R=chrisha@chromium.org

Review URL: https://codereview.appspot.com/133110044

git-svn-id: http://sawbuck.googlecode.com/svn/trunk@2276 15e8cca8-e42c-11de-a347-f34a4f72eb7d