support/script/pkg-stats: handle exception when version comparison fails

With python 3, when a package has a version number x-y-z instead of
x.y.z, then the version returned by LooseVersion can't be compared
which raises a TypeError exception:

Traceback (most recent call last):
  File "./support/scripts/pkg-stats", line 1062, in <module>
    __main__()
  File "./support/scripts/pkg-stats", line 1051, in __main__
    check_package_cves(args.nvd_path, {p.name: p for p in packages})
  File "./support/scripts/pkg-stats", line 613, in check_package_cves
    if pkg_name in packages and cve.affects(packages[pkg_name]):
  File "./support/scripts/pkg-stats", line 386, in affects
    return pkg_version <= cve_affected_version
  File "/usr/lib64/python3.8/distutils/version.py", line 58, in __le__
    c = self._cmp(other)
  File "/usr/lib64/python3.8/distutils/version.py", line 337, in _cmp
    if self.version < other.version:
TypeError: '<' not supported between instances of 'str' and 'int'

This patch handles this exception by adding a new return value when
the comparison can't be done. The code is adjusted to take of this
change. For now, a return value of CVE_UNKNOWN is handled the same way
as a CVE_DOESNT_AFFECT return value, but this can be improved later
on.

Signed-off-by: Gregory CLEMENT <gregory.clement@bootlin.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>