[9.11] fix: dev: Fix buffer overflow in check_zoneconf

The following line caused a buffer overflow due to `tmp` and `len` having the wrong values:
```
                (void)snprintf(tmp, len, "%u/%s", zclass,
                               (ztype == CFG_ZONE_INVIEW) ? target :
                                (viewname != NULL) ? viewname : "_default");
```

Merge branch 'matthijs-fix-buffer-overflow' into 'bind-9.11'

See merge request isc-projects/bind9!11789