ASN1: Fix stack variable overwrite when encoding OID.

ASN1: Fix stack variable overwrite when encoding OID.

The call to sscanf specifies a format string of "%lu", a long unsigned
int.  The pointer argument was cast to unsigned long *, but this is
wrong for 64 bit systems.  On 64 bit systems, unsigned long is 64 bits,
but the oid value is a SilcUInt32 on all systems.  As a result, sscanf
will overwrite a neighboring variable on the stack.  Fix this by
changing the format string to "%u" and removing the cast.