compat/poll/poll.c: Fix a sparse warning

Sparse issues an 'Using plain integer as NULL pointer' warning when
passing the constant '0' as the second parameter in the call to the
WSAEventSelect() function. The function parameter has a pointer type
(WSAEVENT, aka HANDLE, aka void *) so that, in order to suppress the
warning, we simply pass NULL for that parameter in the function call
expression.

Signed-off-by: Ramsay Jones <ramsay@ramsay1.demon.co.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

compat/poll/poll.c

@@ -576,7 +576,7 @@ restart:
 	{
 	  /* It's a socket.  */
 	  WSAEnumNetworkEvents ((SOCKET) h, NULL, &ev);
-	  WSAEventSelect ((SOCKET) h, 0, 0);
+	  WSAEventSelect ((SOCKET) h, NULL, 0);
 
 	  /* If we're lucky, WSAEnumNetworkEvents already provided a way
 	     to distinguish FD_READ and FD_ACCEPT; this saves a recv later.  */