net-im/libpurple: use the SSL_PeerCertificateChain function, instead of SSL_PeerCertificate

The ssl_nss_get_peer_certificates function in libpurple 2.x.y assumes that all intermediate certificates from the peer's presented chain can be found in the NSS certificate DB. This is not the case in NSS 3.103.

This patch is required in order to add a new port for *MS teams* support in pidgin.

This patch replaces a call to ssl_nss_get_peer_certificates by SSL_PeerCertificateChain who retrieves the certificates presented by the SSL peer.SSL_PeerCertificateChain has been in NSS since version 3.15.4 released in 2014.

Additional references: https://bugzilla.mozilla.org/show_bug.cgi?id=1913047

PR: 281761