thread: uci: 解决概率性出现 data race 问题

ThreadSanitizer: data race /home/calcitem/Sanmill/src/thread.cpp:434:18 in Thread::adjustDepth()
ThreadSanitizer: data race /home/calcitem/Sanmill/src/uci.cpp:121:28 in (anonymous namespace)::go(Position*)
ThreadSanitizer: data race /home/calcitem/Sanmill/src/./position.h:285:15 in Position::key() const

src/uci.cpp

@@ -117,8 +117,15 @@ void go(Position *pos)
         }
 #endif
 
-        pos->set(StartFEN, Threads.main());
-        Threads.main()->us = BLACK; // WAR
+        while (true) {
+            if (Threads.main()->searching == true) {
+                continue;
+            }
+
+            pos->set(StartFEN, Threads.main());
+            Threads.main()->us = BLACK; // WAR
+            break;
+        }
 #else
         return;
 #endif