1. Add 'ref_cnt' field to asn1p_expr_t.
2. Initialize 'ref_cnt' field to zero when asn1p_expr_t is allocated.
3. Increase 'ref_cnt' field when asn1p_expr_t is cloned by asn1p_value_fromtype().
4. If 'ref_cnt' field of asn1p_expr_t is larger than zero, then asn1p_expr_free() only decrease its value.
5. Free memory pointed by fields of asn1p_expr_t and itself when 'ref_cnt' is zero and asn1p_expr_free() called.
6. Call asn1p_delete(asn) in main().