Invalid characters cause assert

To trigger the issue, three configuration parameters must have specific settings: "hostname-char-set" must be left at the default setting, which is "[^A-Za-z0-9.-]"; "hostname-char-replacement" must be empty (the default); and "ddns-qualifying-suffix" must *NOT* be empty (the default is empty). DDNS updates do not need to be enabled for this issue to manifest. A client that sends certain option content would then cause kea-dhcp4 to exit unexpectedly. This issue affects Kea versions 3.0.1 through 3.0.1 and 3.1.1 through 3.1.2.