invalid win32 accept() check in ev_pipe

[Vladimir Ivannikov]

Hello,

There is tiny bug in win32 implementation of ev_pipe() caused by the 
fact that accept() returns SOCKET type which is actually UINT_PTR on 
windows which of couse unsigned and can't be
signed compared with 0 (compiler will just optimized this check out)
Patch is trivial:

Index: ev_win32.c
===================================================================
RCS file: /schmorpforge/libev/ev_win32.c,v
retrieving revision 1.17
diff -u -3 -w -r1.17 ev_win32.c
--- ev_win32.c    17 Oct 2015 21:47:56 -0000    1.17
+++ ev_win32.c    10 Nov 2015 20:21:53 -0000
@@ -88,7 +88,7 @@
    if (connect (sock [0], (struct sockaddr *)&addr, addr_size))
      goto fail;

-  if ((sock [1] = accept (listener, 0, 0)) < 0)
+  if ((sock [1] = accept (listener, 0, 0)) == INVALID_SOCKET)
      goto fail;

    /* windows vista returns fantasy port numbers for sockets: