libical: upgrade 3.0.15 -> 3.0.16

Version 3.0.16 is a patch release.

- Fix regressions in 3.0.15 due to improperly tested fuzz fixes
- Fix argument guards in icaltime_as_timet to match documentation and tests.

Signed-off-by: Alexander Kanavin <alex@linutronix.de>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
(cherry picked from commit 77d4557d)
Signed-off-by: Steve Sakoman <steve@sakoman.com>