ECOM-757: Fix reference error with emailOptIn variable

[ECOM-757](https://openedx.atlassian.net/browse/ECOM-757): Uncaught ReferenceError: email_opt_in is not defined

I introduced this bug when resolving merge conflicts by hand.  Unfortunately, this is in the marketing iframe JavaScript, which is currently impossible to unit test.  I have manually verified the fix in devstack (reproduced the issue before, made this change, then the issue was fixed).

Reviewers: @rlucioni @stephensanchez 
FYI: @clintonb