Fix overflow warning in dumb_socketpair() on Win64

The SOCKET type is a pointer, although in practice what's returned really
does look like a file descriptor. It's a low-valued integer such as 0x23
under Wine, 0x54 under Windows 7 in my testing.

The INVALID_SOCKET error return from socket() is defined as (SOCKET)(~0),
or 0xFFFFFFFFFFFFFFFF on Win64. Thus we get errors when assigning it to
an int in the socks[] array. So use -1 here instead.

Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>

compat.c

@@ -295,7 +295,7 @@ int dumb_socketpair(int socks[2], int make_overlapped)
     a.inaddr.sin_addr.s_addr = htonl(INADDR_LOOPBACK);
     a.inaddr.sin_port = 0; 
 
-    socks[0] = socks[1] = INVALID_SOCKET;
+    socks[0] = socks[1] = -1;
     do {
         if (setsockopt(listener, SOL_SOCKET, SO_REUSEADDR, 
                (char*) &reuse, (socklen_t) sizeof(reuse)) == -1)