Remove redundant nTime checks
nTime is always initialized on deserialization or default-initialized
with TIME_INIT, so special casing 0 does not make sense.
Remove redundant nLastTry check
All other places calculate "now - nLastTry", which is safe and correct
to do when nLastTry is 0. So do the same here.
This is a backport of core#25303
Depends on D15088