Start a TT resize only after search finished.

As noticed in the forum, a crash in extract_ponder_from_tt could result
if hash size is set before the ponder move is printed. While it is arguably
a GUI issue (but it got me on the cli), it is easy to avoid this issue.

Closes https://github.com/official-stockfish/Stockfish/pull/1856

No functional change.

src/tt.cpp

@@ -24,6 +24,7 @@
 
 #include "bitboard.h"
 #include "misc.h"
+#include "thread.h"
 #include "tt.h"
 #include "uci.h"
 
@@ -58,6 +59,8 @@ void TTEntry::save(Key k, Value v, Bound b, Depth d, Move m, Value ev) {
 
 void TranspositionTable::resize(size_t mbSize) {
 
+  Threads.main()->wait_for_search_finished();
+
   clusterCount = mbSize * 1024 * 1024 / sizeof(Cluster);
 
   free(mem);