Ensure printing UCI info in multi-pv case

After commit 94b1bbb68be6b0bc3aaf1cb804841a022bcc7007, in case available root moves are less than multiPV, we
could never reach condition:

PVIdx + 1 == multiPV

and as a consequence UCI output is not printed.

Fixed suggested by Joerg Oster.

No functional change.
Merge branch 'master' of https://github.com/official-stockfish/Stockfish