pki: Avoid memory leak on invalid curve NID

Thanks oss-fuzz

https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=24166

Signed-off-by: Jakub Jelen <jjelen@redhat.com>
Reviewed-by: Andreas Schneider <asn@cryptomilk.org>

src/pki.c

@@ -1373,6 +1373,8 @@ static int pki_import_pubkey_buffer(ssh_buffer buffer,
                 nid = pki_key_ecdsa_nid_from_name(ssh_string_get_char(i));
                 SSH_STRING_FREE(i);
                 if (nid == -1) {
+                    ssh_string_burn(e);
+                    SSH_STRING_FREE(e);
                     goto fail;
                 }