Align ponder move and final PV output

[robertnurnberg]

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.

No functional change.

[snicolet]

Do you have a set of positions which could serve as test cases demonstrating the effect(s) of the patch?

[robertnurnberg]

One example is given in the linked issue. Several more can be constructed with a script like this, using the fastchess version from Disservin/fastchess#997.

#!/bin/bash

count=0
while true; do
  ((count++))
  echo "Starting Run #$count..."

  ./fastchess -engine name=sf-dev1 cmd=./stockfish tc=1+0.01 \
  -engine name=sf-dev2 cmd=./stockfish tc=0.2+0.01 \
  -rounds 500 -concurrency -1 -openings file=UHO_Lichess_4852_v1.epd \
  format=epd -check-mate-pvs -check-ponder-pvs \
  -log level=info engine=true file=engine_log.txt append=false > fc_log.txt

  if grep -q "Warning" fc_log.txt; then
    echo "STOPPED: 'Warning' found in fc_log.txt during Run #$count"
    break
  else
    echo "Run #$count clean."
  fi
done

Sample log files: fc_log.txt.gz , engine_log_1m.txt.gz (first 1m lines)

> zgrep Warning fc_log.txt.gz | sed 's/- move.*//' | sort | uniq -c
      2 Warning; Pondermove does not match beginning of last PV 
    713 Warning; PV has length 1 despite ponder move output

The binary from this PR runs cleanly, without any warnings.

Directly looking at the code in master it is possible to reason about all the four cases mentioned in the PR description.