Check result before and bail out if exception has been thrown.

BUG=v8:1172
TEST=test/mjsunit/regress/regress-1172-bis.js

Review URL: http://codereview.chromium.org/6580030

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@6939 ce2b1a6d-e550-0410-aec6-3dcde31c8c00