Fix invalid reuse of weak global handle in GetScriptWrapper.

This fixes a direct usage of a weak global handle in GetScriptWrapper
that just casted it to a strong local handle, while a subsequent GC
might clear it. Handlepocalypse anyone?

R=machenbach@chromium.org
BUG=v8:2988
TEST=mjsunit/regress/regress-2988

Review URL: https://codereview.chromium.org/67273004

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@17622 ce2b1a6d-e550-0410-aec6-3dcde31c8c00