A recent change introduced a cut-off in believing MS's claims that a
zone has always had a yearly cycle of DST, to stop believing it before
1900, as no actual zone adopted DST that early. However, this was only
applied to the data() method to get the data for a specified time;
failure to do the same for {next,previous}Transition lead to
inconsistent results and tripped up an assertion. The change to data()
also failed to consider the situation of a southern hemisphere zone,
for which the rule will claim the early part of the year was in DST;
and init() should also mark an initial DST-recurrence rule as starting
no earlier than 1900.
Add corresponding FIRST_DST_YEAR checks to {next,previous}Transition()
to be consistent with the one in data(), refine data()'s checking and
amend init().
This amends commit 9a83706046
Pick-to: 6.3
Fixes: QTBUG-100873
Change-Id: Ic9822b87caa01561bc0b7d0355963e4c80ef047c
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>
93526430a5