[SPARK-7504] [YARN] NullPointerException when initializing SparkContext in YARN-cluster mode

Added a simple checking for SparkContext.
Also added two rational checking against null at AM object.

Author: ehnalis <zoltan.zvara@gmail.com>

Closes #6083 from ehnalis/cluster and squashes the following commits:

926bd96 [ehnalis] Moved check to SparkContext.
7c89b6e [ehnalis] Remove false line.
ea2a5fe [ehnalis] [SPARK-7504] [YARN] NullPointerException when initializing SparkContext in YARN-cluster mode
4924e01 [ehnalis] [SPARK-7504] [YARN] NullPointerException when initializing SparkContext in YARN-cluster mode
39e4fa3 [ehnalis] SPARK-7504 [YARN] NullPointerException when initializing SparkContext in YARN-cluster mode
9f287c5 [ehnalis] [SPARK-7504] [YARN] NullPointerException when initializing SparkContext in YARN-cluster mode