This fixes an assert when an exception is thrown inside instanceof.

BUG=v8:1207
TEST=mjsunit/regress/regress-1207.js

Review URL: http://codereview.chromium.org/6588083

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@6999 ce2b1a6d-e550-0410-aec6-3dcde31c8c00