JIRA: https://issues.apache.org/jira/browse/SPARK-7800

`isDefined` is marked as true twice in `Location.putNewKey`. The first one is unnecessary and will cause problem because it is too early and before some assert checking. E.g., if an attempt with incorrect `keyLengthBytes` marks `isDefined` as true, the location can not be used later.

ping JoshRosen

Author: Liang-Chi Hsieh <viirya@gmail.com>

Closes #6324 from viirya/dup_isdefined and squashes the following commits:

cbfe03b [Liang-Chi Hsieh] isDefined should not marked too early in putNewKey.