BUG/MEDIUM: fd: Don't fd_stop_recv() a fd we don't own.

In fd_takeover(), if we failed to grab the fd, when a double-width
compare-and-swap is not implemented, do not call fd_stop_recv() on the
fd, it is not ours and may be used by another thread.

src/fd.c

@@ -362,7 +362,8 @@ end:
 	 * the fd is polled by the thread that used to own it, the new thread
 	 * is supposed to call subscribe() later, to activate polling.
 	 */
-	fd_stop_recv(fd);
+	if (ret != -1)
+		fd_stop_recv(fd);
 	return ret;
 #else
 	unsigned long old_masks[2];