threads::shared "$#shared = N" off-by-one error

threads::shared "$#shared = N" off-by-one error

RT #122950

    my @a : shared;
    $#a = 3;  # actually set it to 4

There was a simple off-by-one error in the XS code that handled the
STORESIZE tie method (confusing the array size and fill, which differ
by 1).

Amazingly, there was no test for it, and no-one had noticed up until now.

Note that this commit causes three tests in object2.t to fail: this
is because fixing the $#shared bug exposed another bug that was being
masked by this one. They will be fixed in the next commit