1.5.0-P1: configuration error - Attempt to parse truncated vendor option

name: 1.5.0-P1: configuration error - Attempt to parse truncated vendor option

Describe the bug The following configuration error is generated when defining option 125 encapsulated option space:
2020-12-02 00:20:23.376 ERROR [kea-dhcp4.dhcp4/130] DHCP4_CONFIG_LOAD_FAIL configuration error using file: kea-dhcp4.conf.test, reason: option data does not match option definition (space: vendor-encapsulated-options-space, code: 125): Attempt to parse truncated vendor option (kea-dhcp4.conf.test:42:19)
2020-12-02 00:20:23.376 ERROR [kea-dhcp4.dhcp4/130] DHCP4_INIT_FAIL failed to initialize Kea server: configuration error using file 'kea-dhcp4.conf.test': option data does not match option definition (space: vendor-encapsulated-options-space, code: 125): Attempt to parse truncated vendor option (kea-dhcp4.conf.test:42:19)

Customer reports that on investigation the problem occurs at:
src/lib/dhcp/libdhcp++.cc source, function unpackVendorOptions4 when parsing option value.
It would appear that for some reason 43.125 is treated as just 125 (parent option) and the vendor ID (4-byte unsigned integer) + length octet + data is expected – and that is where error is thrown. This function is called from OptionVendor class, method unpack in option_vendor.cc source.

See RT #17358

To Reproduce
The following configuration:

{   
    "Dhcp4": {  
        "valid-lifetime": 3600,  
        "echo-client-id": true,  
        "interfaces-config": {  
            "interfaces": [  
                "*"  
            ],  
            "re-detect": true,  
            "dhcp-socket-type": "raw",  
            "outbound-interface": "same-as-inbound"  
        },   
        "control-socket": {
            "socket-type": "unix",  
            "socket-name": "/tmp/kea4_command"
        },  
        "lease-database": {  
            "type": "memfile",  
            "persist": true,  
            "name": "/tmp/dhcp4.leases",  
            "lfc-interval": 3600  
        },  
        "sanity-checks": {  
            "lease-checks": "fix"  
        },  
        "subnet4": [  
            {  
                "subnet": "10.0.20.0/24",  
                "match-client-id": true  
            }  
        ],  
    "option-def": [  
        {  
            "name": "cookie",  
            "code": 125,  
            "type": "string",  
            "space": "ABC"  
        }  
    ],  
    "client-classes": [  
        {  
            "name": "ABC",  
            "test": "(option[vendor-class-identifier].text == 'ABC')",  
            "option-def": [  
                {  
                    "name": "vendor-encapsulated-options",  
                    "code": 43,  
                    "type": "empty",  
                    "encapsulate": "ABC"  
                }
            ],
            "option-data": [  
                {  
                    "name": "cookie",  
                    "space": "ABC",  
                    "data": "1ABCDE"  
                },  
                {  
                    "name": "vendor-encapsulated-options"  
                }  
            ]  
        }  
    ]  
},  
    "Logging": {  
        "loggers": [  
            {  
                "name": "kea-dhcp4",  
                "output_options": [  
                    {  
                        "output": "/tmp/kea_log"  
                    }  
                ],  
                "severity": "DEBUG",  
                "debuglevel": 99  
            }  
          ]  
         }  
}

Also note that using a 4 byte string (for example "1APC") does allow Kea to start up