Fix false TT hits for zero verification keys

[kuel27]

When a TT slot is cleared, it has a verification key of zero, but zero is also a valid 21-bit verification key. So a probe with this key can match an unused slot and read zeroed data as a valid TT entry.

This reproduces the bug:

let tt = TranspositionTable::default();
assert!(tt.read(0xed3e785687200000, 0, 0).is_none());

The assertion fails before this patch and passes after it. Fixing the false hit changes the standard bench from 2985186 to 2957947 nodes.

To fix this, reserve verification key zero for unused slots. Skip TT reads and writes for positions with this key.

Bench: 2957947

[peregrineshahin]

Should fix that by using some other mechanism to determine tthits than keys, usually people reserve internal entry depth of 0 to refer to no entry but that has some complications in Reckless where a real entry could also have that depth.. I don’t think this version of the PR will do.