ASN1: Fix NULL pointer dereference on stack allocation failure.

ASN1: Fix NULL pointer dereference on stack allocation failure.

If the second silc_stack_alloc fails, then asn1->stack2 is NULL.  Thus,
when silc_asn1_init calls silc_stack_free(asn1->stack2), it is
equivalent to silc_stack_free(NULL).  However, silc_stack_free does not
check for a NULL pointer.  Fix silc_asn1_init to free asn1->stack1, as
was intended.