[x509] Ensure certificate remains valid during x509_append()

[x509] Ensure certificate remains valid during x509_append()

The allocation of memory for the certificate chain link may cause the
certificate itself to be freed by the cache discarder, if the only
current reference to the certificate is held by the certificate store
and the system runs out of memory during the call to malloc().

Ensure that this cannot happen by taking out a temporary additional
reference to the certificate within x509_append(), rather than
requiring the caller to do so.

Signed-off-by: Michael Brown <mcb30@ipxe.org>