The assumption in asn1_type() that an ASN.1 cursor will always contain
a type byte is incorrect.  A cursor that has been cleanly invalidated
via asn1_invalidate_cursor() will contain a type byte, but there are
other ways in which to arrive at a zero-length cursor.

Fix by explicitly checking the cursor length in asn1_type().  This
allows asn1_invalidate_cursor() to be reduced to simply zeroing the
length field.

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