GTlsConnection: fix leak of the certificate chain

g_set_object() increases the reference count on the setting object, but retrieve_peer_certificate() does a full transfer of the returned certificate already, hence there's an extra reference that is never decreased.

Valgrind output:

==30427== 52,009 (72 direct, 51,937 indirect) bytes in 1 blocks are definitely lost in loss record 2,881 of 2,894
==30427==    at 0x483577F: malloc (vg_replace_malloc.c:309)
==30427==    by 0x49581F5: g_malloc (gmem.c:99)
==30427==    by 0x497344F: g_slice_alloc (gslice.c:1024)
==30427==    by 0x497348E: g_slice_alloc0 (gslice.c:1050)
==30427==    by 0x4A6AB69: g_type_create_instance (gtype.c:1849)
==30427==    by 0x4A50A85: g_object_new_internal (gobject.c:1827)
==30427==    by 0x4A51928: g_object_new_valist (gobject.c:2152)
==30427==    by 0x4A50686: g_object_new (gobject.c:1670)
==30427==    by 0x84FEBEB: g_tls_certificate_gnutls_new (gtlscertificate-gnutls.c:393)
==30427==    by 0x84FF777: g_tls_certificate_gnutls_build_chain (gtlscertificate-gnutls.c:719)
==30427==    by 0x85028B8: g_tls_connection_gnutls_retrieve_peer_certificate (gtlsconnection-gnutls.c:811)
==30427==    by 0x8508C93: update_peer_certificate_and_compute_errors (gtlsconnection-base.c:1237)
==30427==    by 0x8508D9F: accept_or_reject_peer_certificate (gtlsconnection-base.c:1260)
==30427==    by 0x495224C: g_idle_dispatch (gmain.c:5617)
==30427==    by 0x494F6F2: g_main_dispatch (gmain.c:3179)
==30427==    by 0x495055A: g_main_context_dispatch (gmain.c:3844)