Fix a MSVC warning

Not correct warning about use of an uninitialized
variable. The warning is not correct becuase we can
never reach the warned code when in SpNode, anyhow
the fix is simple.

No functional change.

Signed-off-by: Marco Costalba <mcostalba@gmail.com>

src/search.cpp

@@ -564,6 +564,7 @@ namespace {
     {
         tte = NULL;
         ttMove = excludedMove = MOVE_NONE;
+        ttValue = VALUE_ZERO;
         sp = ss->sp;
         bestMove = sp->bestMove;
         threatMove = sp->threatMove;