Fix upperbound/lowerbound output in multithreaded case
In case a stop is received during multithreaded searches, the PV of the best
thread might be printed without the correct upperbound/lowerbound indicators.
This was due to the pvIdx variable being incremented after receiving the stop.
passed STC:
https://tests.stockfishchess.org/tests/view/666985da602682471b064d08
LLR: 2.93 (-2.94,2.94) <-1.75,0.25>
Total: 196576 W: 51039 L: 50996 D: 94541
Ptnml(0-2): 760, 22545, 51603, 22652, 728
closes https://github.com/official-stockfish/Stockfish/pull/5391
Bench: 1160467