list_tls_signatures(): Avoid leak with zero length builtin_sigalgs

Fixes Coverity 1616307

Reviewed-by: Neil Horman <nhorman@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/25219)