[SPARK-7800] isDefined should not marked too early in putNewKey

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.

unsafe/src/main/java/org/apache/spark/unsafe/map/BytesToBytesMap.java

@@ -429,7 +429,6 @@ public final class BytesToBytesMap {
         long valueBaseOffset,
         int valueLengthBytes) {
       assert (!isDefined) : "Can only set value once for a key";
-      isDefined = true;
       assert (keyLengthBytes % 8 == 0);
       assert (valueLengthBytes % 8 == 0);
       if (size == MAX_CAPACITY) {