Align ponder move and final PV output
In master for various reasons the given ponder move may get out of sync
with (the second move of) the last PV sent to the GUI:
1. If a ponder move is extracted from the TT despite bestmove leading to
a game ending draw.
2. If a ponder move is extracted from the TT from a fail high/low, since
after the ponder move extraction no new PV is sent to the GUI.
3. If `syzygy_extend_pv()` from within the call to `pv()` changes the PV
after the ponder move has been selected.
4. If the PV roll-back to protect mated-in scores differs in the second
move from the final PV sent to the GUI.
This PR keeps ponder move and the last printed PV in sync.
Fixes #6676.
closes https://github.com/official-stockfish/Stockfish/pull/6679
No functional change