Even though mktime() is documented as ignoring the tm_wday field, the
coverity static analyzer complains about passing an uninitialized value.
Set the field to zero to make it happy.
These were found by Gentoo's QA while rebuilding the world with
clang-16: https://bugs.gentoo.org/880519
Signed-off-by: Holger Hoffstätte <holger@applied-asynchrony.com>
