Windows handle sockets: fix error handling in sentdata().

In the sentdata callback function given to handle_output_new, the
'new_backlog' parameter can be negative, and if so, it represents a
Windows error code and not a backlog size at all. handle_sentdata was
not checking for this before passing it on to plug_sent.

windows/winhsock.c

@@ -95,6 +95,14 @@ static void handle_sentdata(struct handle *h, int new_backlog)
 {
     Handle_Socket ps = (Handle_Socket) handle_get_privdata(h);
 
+    if (new_backlog < 0) {
+        /* Special case: this is actually reporting an error writing
+         * to the underlying handle, and our input value is the error
+         * code itself, negated. */
+        plug_closing(ps->plug, win_strerror(-new_backlog), -new_backlog, 0);
+        return;
+    }
+
     plug_sent(ps->plug, new_backlog);
 }