[ISC-support #19987] Race condition on the initialization of flex_id_expr[4|6]

https://support.isc.org/Ticket/Display.html?id=19987

Customer reports seeing exceptions in the logs:

Dec 15 13:26:31 dhcp1 kea-dhcp-safe-start.sh: 2021-12-15 13:26:31.358 ERROR [kea-dhcp4.callouts/11189.139755113694976] HOOKS_CALLOUT_EXCEPTION exception thrown by callout on hook pkt4_receive registered by library with index 4 (callout address 0x7f1b46695e60): expression: [relay4[2].hex] error: <string>:1.1-7: syntax error, unexpected [ (callout duration: 0.310 ms)
Dec 15 13:26:31 dhcp1 kea-dhcp-safe-start.sh: 2021-12-15 13:26:31.358 ERROR [kea-dhcp4.callouts/11189.139755122087680] HOOKS_CALLOUT_EXCEPTION exception thrown by callout on hook pkt4_receive registered by library with index 4 (callout address 0x7f1b46695e60): expression: [relay4[2].hex] error: <string>:1.1-6: syntax error, unexpected relay4, expecting top-level bool or top-level string (callout duration: 0.315 ms)
Dec 15 14:27:36 dhcp1 kea-dhcp-safe-start.sh: 2021-12-15 14:27:36.383 ERROR [kea-dhcp4.callouts/16176.140013766174464] HOOKS_CALLOUT_EXCEPTION exception thrown by callout on hook pkt4_receive registered by library with index 4 (callout address 0x7f577f4ce340): expression: [relay4[2].hex] error: <string>:1.8-19: syntax error, unexpected relay4, expecting top-level bool or top-level string (callout duration: 0.352 ms)

Razvan stated he had found a race: https://mattermost.isc.org/isc/pl/d1aye5oqgp8xbehpks41r6zkyo