fuzz/oss-fuzz/build_samba: fetch fuzz seeds

fuzz/oss-fuzz/build_samba: fetch fuzz seeds

There is a git repository at
https://gitlab.com/samba-team/samba-fuzz-seeds that contains the
seeds. When the master branch of that repository is updated, a CI job
runs that creates a zip file of all the seeds as an artifact. That zip
file is downloaded and unpacked by oss_fuzz/build_samba. The contents
of that zip are further zips that contain the seeds for each fuzzing
binary; these are placed next to the binaries in the manner that
oss-fuzz expects.

That is, beside 'fuzz_foo', we put 'fuzz_foo_seed_corpus.zip' which
contains a pile of fuzz_foo seeds.

There may be times when a new fuzz target does not have a seed corpus,
and times when a removed fuzz target leaves behind a seed corpus.
This is OK, so we don't insist on an exact match between the target
names and the zip names, only that there is some overlap.

Signed-off-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>
Autobuild-User(master): Andrew Bartlett <abartlet@samba.org>
Autobuild-Date(master): Wed Oct 21 03:47:35 UTC 2020 on sn-devel-184