unit test cb_ctl_dhcp_unittest.cc fails and crashes on freebsd

Machine: freebsd113-64

Log fragment:

cb_ctl_dhcp_unittest.cc:457: Failure
Value of: found_network
  Actual: false
Expected: true
Assertion failed: (px != 0), function operator->, file /usr/local/include/boost/smart_ptr/shared_ptr.hpp, line 734.

Full logs:

• https://jenkins.isc.org/job/kea-master-ut-basic/531/parsed_console/
• https://jenkins.isc.org/job/kea-1.6/job/ut-basic/532/execution/node/197/log/?consoleFull