Fix some not-always-turned-on compiler warnings

Hi!

These are the issues I had to fix to get scrypt-jane to build under Tor's set of compiler warnings.

In a couple of cases, you might want an alternative approach instead:

Instead of 6bfcf80, you may want to add a prototype for scrypt_ensure_zero.

Instead of 9bdcf60, you might want to change the name in the header rather than the name of the function.