bibtex: Robustify code extracting year and month from date

Previous implementation could lead to errors when an invalid date is present in source codemeta.

Fix citation generation for https://archive.softwareheritage.org/browse/origin/directory/?origin_url=https://github.com/htcondor/htcondor

Jenkins job DCIDX/gitlab-builds #242 succeeded in 2 min 3 sec.
See Console Output, Blue Ocean and Coverage Report for more details.

approved this merge request

merged manually