CVE-2019-915{1,2}, -8397: add upstream tracker links

Add links to upstream bug tracker entries for hdf5 CVE-2019-9152,
CVE-2019-9151 and CVE-2019-8397 (other -2019 CVEs don't have bug
tracker entries).

Note that access to the bug tracker requires login. Annoying, but you
can create a free account quickly following

https://www.hdfgroup.org/community/community-development/