ssl: fix OOB write in SSL_get_shared_ciphers when no shared ciphers

ssl: fix OOB write in SSL_get_shared_ciphers when no shared ciphers

When no cipher names are appended, p remains at buf and the unconditional
p[-1] = '\0' underflows. Only NUL-terminate if at least one cipher was written;
otherwise return an empty string safely.

Signed-off-by: Joshua Rogers <MegaManSec@users.noreply.github.com>
Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/28785)