[SPARK-18986][CORE] ExternalAppendOnlyMap shouldn't fail when forced to spill...

[SPARK-18986][CORE] ExternalAppendOnlyMap shouldn't fail when forced to spill before calling its iterator

## What changes were proposed in this pull request?

`ExternalAppendOnlyMap.forceSpill` now uses an assert to check if an iterator is not null in the map. However, the assertion is only true after the map is asked for iterator. Before it, if another memory consumer asks more memory than currently available, `ExternalAppendOnlyMap.forceSpill` is also be called too. In this case, we will see failure like this:

    [info]   java.lang.AssertionError: assertion failed
    [info]   at scala.Predef$.assert(Predef.scala:156)
    [info]   at org.apache.spark.util.collection.ExternalAppendOnlyMap.forceSpill(ExternalAppendOnlyMap.scala:196)
    [info]   at org.apache.spark.util.collection.Spillable.spill(Spillable.scala:111)
    [info]   at org.apache.spark.util.collection.ExternalAppendOnlyMapSuite$$anonfun$13.apply$mcV$sp(ExternalAppendOnlyMapSuite.scala:294)

This fixing is motivated by http://apache-spark-developers-list.1001551.n3.nabble.com/java-lang-AssertionError-assertion-failed-tc20277.html.

## How was this patch tested?

Jenkins tests.

Please review http://spark.apache.org/contributing.html before opening a pull request.

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

Closes #16387 from viirya/fix-externalappendonlymap.