Fix "failed to makeProblem too many times" bug

Test Plan:
1. Get a repro...
1a. In webapp, do radical_equations problems over and over until it fails.
1b. Typing `KhanUtil.tmpl.getVAR("SOLUTION")` in the console is really helpful.
1c. In my case it failed on seed 63 (`Khan.getSeedInfo()` in the console to verify).
2. Make these changes.
3. Reload the page and get a problem!
4. Do `Khan.getSeedInfo()` to see that it just gave me seed 64.
4. Load http://exercises.ka.local/exercises/radical_equations.html?debug&problem=0&seed=63 in local mode to see the same behavior of moving to the next seed.

Reviewers: benkomalo

Reviewed By: benkomalo

Differential Revision: http://phabricator.khanacademy.org/D9442