fix: dev: fix possible NULL dereference in `cfg_map_findclause()`

`cfg_map_findclause()` did not check whether a clause existed before dereferencing it, which could lead to a NULL dereference. Add the missing check to prevent this.

In practice, this was not triggering any known bug, since `cfg_map_findclause()` is only called in contexts where the clause is known to exist.

Closes #5997

Merge branch '5997-findclause' into 'main'

See merge request isc-projects/bind9!12052