res_pjsip_authenticator_digest: Fix issue with missing auth and DONT_OPTIMIZE

res_pjsip_authenticator_digest: Fix issue with missing auth and DONT_OPTIMIZE

The return code fom digest_check_auth wasn't explicitly being initialized.
The return code also wasn't explicitly set to CHALLENGE when challenges
were sent.  When optimization was turned off (DONT_OPTIMIZE), the compiler
was setting it to "0"(CHALLENGE) which worked fine.  However, with
optimization turned on, it was setting it to "1" (SUCCESS) so if there was
no incoming Authorization header, the function was returning SUCCESS to the
distributor allowing the request to incorrectly succeed.

The return code is now initialized correctly and is now explicitly set
to CHALLENGE when we send challenges.