Add defensive assert against a weak callback on an empty persistent.

(This shouldn't happen in the first place, as by definition that callback would never be called. However, the referenced bug sorta looks like this happened. If so, the CHECK should help us pinpoint the culprit.)

R=jochen@chromium.org
BUG=368095
LOG=N

Review URL: https://codereview.chromium.org/265823006

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21521 ce2b1a6d-e550-0410-aec6-3dcde31c8c00