MB-100: minor fix in _check_valid_keys

fix UnboundLocalError: local variable 'key_vals' referenced before assignment
after we self.set_exception(error)

Change-Id: I51fc093d310386ea75ddebdb4f67adcdb9d1c9fa
Reviewed-on: http://review.couchbase.org/24324
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Andrei Baranouski <andrei.baranouski@gmail.com>