Attempt to silence untrusted loop bound

Attempt to silence untrusted loop bound

Assign hit_len + key_len to len and test the result
rather than recomputing and letting the compiler simplify.

    213        isc_region_consume(&region, 2); /* hit length + algorithm */
        9. tainted_return_value: Function uint16_fromregion returns tainted data. [show details]
        10. tainted_data_transitive: Call to function uint16_fromregion with tainted argument *region.base returns tainted data.
        11. tainted_return_value: Function uint16_fromregion returns tainted data.
        12. tainted_data_transitive: Call to function uint16_fromregion with tainted argument *region.base returns tainted data.
        13. var_assign: Assigning: key_len = uint16_fromregion(&region), which taints key_len.
    214        key_len = uint16_fromregion(&region);
        14. lower_bounds: Casting narrower unsigned key_len to wider signed type int effectively tests its lower bound.
        15. Condition key_len == 0, taking false branch.
    215        if (key_len == 0) {
    216                RETERR(DNS_R_FORMERR);
    217        }
        16. Condition !!(_r->length >= _l), taking true branch.
        17. Condition !!(_r->length >= _l), taking true branch.
    218        isc_region_consume(&region, 2);
        18. lower_bounds: Casting narrower unsigned key_len to wider signed type int effectively tests its lower bound.
        19. Condition region.length < (unsigned int)(hit_len + key_len), taking false branch.
    219        if (region.length < (unsigned)(hit_len + key_len)) {
    220                RETERR(DNS_R_FORMERR);
    221        }
    222
        20. lower_bounds: Casting narrower unsigned key_len to wider signed type int effectively tests its lower bound.
        21. Condition _r != 0, taking false branch.
    223        RETERR(mem_tobuffer(target, rr.base, 4 + hit_len + key_len));
        22. lower_bounds: Casting narrower unsigned key_len to wider signed type int effectively tests its lower bound.
        23. var_assign_var: Compound assignment involving tainted variable 4 + hit_len + key_len to variable source->current taints source->current.
    224        isc_buffer_forward(source, 4 + hit_len + key_len);
    225
    226        dns_decompress_setmethods(dctx, DNS_COMPRESS_NONE);

    CID 281461 (#1 of 1): Untrusted loop bound (TAINTED_SCALAR)
        24. tainted_data: Using tainted variable source->active - source->current as a loop boundary.
    Ensure that tainted values are properly sanitized, by checking that their values are within a permissible range.
    227        while (isc_buffer_activelength(source) > 0) {
    228                dns_name_init(&name, NULL);
    229                RETERR(dns_name_fromwire(&name, source, dctx, options, target));
    230        }

(cherry picked from commit 2f946c831a5ad4ed5c2694c51531dd4efa3b7aed)