util/sss_krb5.c: elimination of unreachable code
It was wrong to check `kt_err` after
```
if (!principal_found) {
...
goto done;
}
```
since getting to this point of code would mean `kt_err` equals to 0 and
thus statement inside `if (kt_err != 0) ...` was unreachable.
Moreover it was logical error to do `goto done;` inside this statement
without setting `kerr`.
Reviewed-by: Sumit Bose <sbose@redhat.com>