ntp: reset tentative flag only when sample was accumulated

When selecting sources from a pool, ignore responses which didn't
produce a new sample. Sources with acceptable delay (as configured by
the maxdelay* options) should be prefered.

ntp_core.c

@@ -1519,7 +1519,7 @@ receive_packet(NTP_Packet *message, struct timeval *now, double now_err, NCR_Ins
             pkt_root_delay, pkt_root_dispersion);
   }            
 
-  return valid_packet;
+  return good_packet;
 }
 
 /* ================================================== */