res_pjsip_outbound_authenticator_digest.c: Fix memory pool leak.

res_pjsip_outbound_authenticator_digest.c: Fix memory pool leak.

Responding to authentication challenges leaks PJSIP memory pools.

The leak was introduced with a pjproject 2.5.5 API change.
https://trac.pjsip.org/repos/ticket/1929 changed the API usage of
pjsip_auth_clt_init() to require the new API pjsip_auth_clt_deinit() to
clean up cached authentication allocations that get allocated with
pjsip_auth_clt_reinit_req().

ASTERISK-26516 #close

Change-Id: I4473141b8c3961d0dc91c382beb3876b3efb45c8