after a certain point time_map should not be zero

While porting tlsdate to Debian GNU/Hurd 7.0 I noticed that we would return
zero as a valid server time. This happens as a result of some strange mmap and
memcpy interactions on the Hurd. This could happen if the server was a false ticker
or as a result of some other strange flukes, so we should never accept it and
if time_map is zero, we should stop trying to use this server. We die when we
encounter zero and it is probably the case that a smart administrator who
wanted to mess with tlsdate users would return 32bits of zeros. We consider
this useful as it is wrong and detectable as such.
We care about the resulting server_time_s rather than time_map being 0