Function init_thread() should return an integer under Windows

It happens that NULL is 0, but the conventional meaning is of
a zero pointer, so repleace with an explicit 0 integer value.

No functional change.

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

src/search.cpp

@@ -2593,7 +2593,7 @@ namespace {
   DWORD WINAPI init_thread(LPVOID threadID) {
 
     TM.idle_loop(*(int*)threadID, NULL);
-    return NULL;
+    return 0;
   }
 
 #endif