Fix assertion when collecting stack trace from debug break.

R=mstarzinger@chromium.org
BUG=v8:3296
LOG=N

Review URL: https://codereview.chromium.org/256113003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21015 ce2b1a6d-e550-0410-aec6-3dcde31c8c00