Fixed memory leak with threads on win32

Signed-off-by: Pieter Hintjens <ph@imatix.com>

src/thread.cpp

@@ -47,6 +47,8 @@ void zmq::thread_t::stop ()
 {
     DWORD rc = WaitForSingleObject (descriptor, INFINITE);
     win_assert (rc != WAIT_FAILED);
+    BOOL rc2 = CloseHandle (descriptor);
+    win_assert (rc != 0);
 }
 
 #else