[9.20] fix: nil: silence tainted scalar in client.c

Coverity detected that 'optlen' was not being checked in 'process_opt'.
This is actually already done when the OPT record was initially
parsed.  Add an INSIST to silence Coverity as is done in message.c.

Closes #5330

Backport of MR !10500

Merge branch 'backport-5330-tainted-scalar-in-client-c-9.20' into 'bind-9.20'

See merge request isc-projects/bind9!10505