If the notification message length is 0, ignore the message (#4237)

Fixes #4234.

Signed-off-by: Jorge Niedbalski <jnr@metaklass.org>

src/core/manager.c

@@ -1721,6 +1721,10 @@ static int manager_dispatch_notify_fd(sd_event_source *source, int fd, uint32_t
 
                 return -errno;
         }
+        if (n == 0) {
+                log_debug("Got zero-length notification message. Ignoring.");
+                return 0;
+        }
 
         CMSG_FOREACH(cmsg, &msghdr) {
                 if (cmsg->cmsg_level == SOL_SOCKET && cmsg->cmsg_type == SCM_RIGHTS) {