Fix broken UCI 'wait for stop'

When we reach the maximum depth, we can finish the
search without a raise of Signals.stop. However, if
we are pondering or in an infinite search, the UCI
protocol states that we shouldn't print the best move
before the GUI sends a "stop" or "ponderhit" command.

It was broken by lazy smp. Fix it by moving the stopping
of the threads after waiting for GUI.

No functional change.