CDRIVER-4699 fix leak on repeated attempts to authenticate (#1364)

* CDRIVER-4699 add regression test

* Call `_mongoc_scram_destroy` when resetting auth state

* only call if `MONGOC_ENABLE_CRYPTO` is defined

`_mongoc_scram_destroy` is conditionally defined. If `MONGOC_ENABLE_CRYPTO` is not defined, scram auth is not supported.