[PATCH 2/2] login-common: Avoid array_front() panic on empty forward_fields array

[PATCH 2/2] login-common: Avoid array_front() panic on empty forward_fields array

sasl_server_auth_request_info_fill() and proxy_redirect_reauth()
NUL-terminate the forward_fields array via array_append_zero() +
array_pop_back() and then call array_front() to hand the C array to
the auth client. array_front() asserts when the array is empty, so a
created-but-empty forward_fields array crashes login.

The empty-array case is no longer reachable from
client_forward_decode_base64() after the previous commit, but guard
defensively here as well: any future caller that creates the array
without populating it should not be able to panic the process.

Gbp-Pq: Name 0002-login-common-Avoid-array_front-panic-on-empty-forwar.patch