[bugfix] Send correct search depth in final UCI info line
For quite some time SF has reported a too low final search depth in rare cases
(in multi-threaded search if a non-main thread was selected). Unfortunately,
the recent refactoring in #6704 means that this now happens for almost every
stopped search.
This PR fixes this issue, which was first spotted and reported by joergoster,
by ensuring that `rootDepth` always holds the last depth for which a search was
started.
Fixes #6756.
closes https://github.com/official-stockfish/Stockfish/pull/6761
No functional change.