io.t test failure

[Simon Glover]

 Test number 6 in io.t is failing on a number of the tinderboxes with
 a double-free error.

 Test 6 does this:

    open P0, "temp.file", "<"
    clone P1, P0
    read S0, P1, 1024
    print S0
    end

 And the clone function in the ParrotIO vtable does this:

    void clone (PMC *dest) {
        VTABLE_init(INTERP, dest);
-->     /* For now both PMCs refer to the same ParrotIO object.
         * If we have different IO layers, we might copy these structures
         */
        PMC_data(dest) = PMC_data(SELF);
        dest->cache.struct_val = SELF->cache.struct_val;
    }

 (pay particular attention to the line with the arrow)

 I would guess that we're getting the double-free because we're assuming
 that ParrotIO PMCs point at distinct objects, but I don't have time to
 track this down any further tonight, so I'm posting this to the list
 before I forget...

 Simon