[network-data] check serverTlv data length before parsing (#6052)
[network-data] check serverTlv data length before parsing (#6052)

This commit adds a check in `GetBackboneRouterPrimary()` to verify
that the server sub-TLV data length is larger than a expected minimum
before casting it to `BackboneRouterServerData` and parsing/reading the
info.
openthread/openthread,srickardti/openthread,abtink/openthread,abtink/openthread,openthread/openthread,srickardti/openthread,srickardti/openthread,srickardti/openthread,openthread/openthread,jwhui/openthread,abtink/openthread,jwhui/openthread,jwhui/openthread,abtink/openthread,openthread/openthread,jwhui/openthread