fix uninitialised memory use in isakmpd

x509_cert_get_subjects() in isakmpd/x509.c has the following code:

(*id)[0] = malloc((*id_len)[0]);
(...)
SET_ISAKMP_ID_TYPE((*id)[0] - ISAKMP_GEN_SZ, IPSEC_ID_DER_ASN1_DN);

this causes only the first byte of (*id)[0] to be filled; the remaining
three in the first word are left uninitialised. they don't seem to be
inspected anywhere, nor are they transmitted to peers, so this bug is
hopefully harmless.

ok natano@