resolved fdo#80166 check input against date acceptance pattern plausibility

... to prevent confusion of #.### input with D.M that then later is
discarded as invalid date input instead of accepted as valid numeric
input.

(cherry picked from commit 836e504c859a5b67f7ab7ba842785951d41058cd)

work around nonsense -Werror=maybe-uninitialized, fdo#80166 follow-up

(cherry picked from commit 397362d8532d7b0abe38f2024dd2cefe2482d6a3)

0f9cf74550e43d174bf6ac75e70c51ab7f51ccf8

Change-Id: I178e28b7ef3b26d04eecc73e5e5c61ee41f89e32
Reviewed-on: https://gerrit.libreoffice.org/10036
Reviewed-by: David Tardon <dtardon@redhat.com>
Tested-by: Markus Mohrhard <markus.mohrhard@googlemail.com>
Reviewed-by: Markus Mohrhard <markus.mohrhard@googlemail.com>