util: Avoid potential uninitialized read in FormatISO8601DateTime(int64_t nTime) by checking gmtime_s/gmtime_r return value

Summary:

The same goes for other very large positive and negative arguments.

Fix by simply checking the gmtime_s/gmtime_r return value :)

Backport of core PR18162.

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, PiRK

Reviewed By: PiRK

Differential Revision: https://reviews.bitcoinabc.org/D8219