Fix crash with duplicated initstepslew address

Fix crash with duplicated initstepslew address

When there are duplicate ntp servers listed on the initstepslew line, 2
SourceRecords are created (sourceA and sourceB), and two timers are
created (timerA and timerB).  When ntp responses are received, only
sourceA is updated because of the way read_from_socket searches for a
matching record.  Eventually, the criteria for sourceA are met, causing
timerA to stop and n_completed_sources to increment.  timerB continues
to trigger, sending ntp poll messages to the ntp server.  Responses from
that server are assigned to sourceA, triggering the criteria for sourceA
and causing n_completed_sources to increment improperly.  Once this
happens enough times, n_complete_sources == number of servers and all
SourceRecords are deleted.  The next time timerB triggers, it attempts
to access sourceB, which was already been deleted, causing the core.