Fix missing exception check in typed array constructor.

The typed array constructor might fail if the first argument is an
object with a length property. Accessing the property can cause an
exception to be thrown and an explicit check needs to be performed.

R=verwaest@chromium.org
BUG=chromium:168545
TEST=mjsunit/regress/regress-crbug-168545.js

Review URL: https://codereview.chromium.org/11777014

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@13325 ce2b1a6d-e550-0410-aec6-3dcde31c8c00