CID 510858: Null pointer dereferences (FORWARD_NULL) in check_keys()

Coverity Scan report for a new test:

/bin/tests/system/ksr/tests_ksr.py: 122 in check_keys()
116             if num == 0:
117                 active = created
118             else:
119                 active = retired
120     
121             # published: dnskey-ttl + publish-safety + propagation
>>>     CID 510858:  Null pointer dereferences  (FORWARD_NULL)
>>>     Invalid operation on null-like value "active".
122             published = active - timedelta(hours=2, minutes=5)
123     
124             # retired: zsk-lifetime
125             if lifetime is not None:
126                 retired = active + lifetime
127                 # removed: ttlsig + retire-safety + sign-delay + propagation

The logic in Coverity Scan seems to be as follows: lifetime is None, retired = None, in the subsequent iteration active = retired, and finally: published = active - timedelta(hours=2, minutes=5).