[SPARK-17245][SQL][BRANCH-1.6] Do not rely on Hive's session state to retrieve HiveConf

## What changes were proposed in this pull request?
Right now, we rely on Hive's `SessionState.get()` to retrieve the HiveConf used by ClientWrapper. However, this conf is actually the HiveConf set with the `state`. There is a small chance that we are trying to use the Hive client in a new thread while the global client has not been created yet. In this case, `SessionState.get()` will return a `null`, which causes a NPE when we call `SessionState.get(). getConf `. Since the conf that we want is actually the conf we set to `state`. I am changing the code to just call `state.getConf` (this is also what Spark 2.0 does).

## How was this patch tested?
I have not figured out a good way to reproduce this.

Author: Yin Huai <yhuai@databricks.com>

Closes apache#14816 from yhuai/SPARK-17245.

Author: yhuai

sql/hive/src/main/scala/org/apache/spark/sql/hive/client/ClientWrapper.scala

@@ -213,7 +213,7 @@ private[hive] class ClientWrapper(
   }
 
   /** Returns the configuration for the current session. */
-  def conf: HiveConf = SessionState.get().getConf
+  def conf: HiveConf = state.getConf
 
   override def getConf(key: String, defaultValue: String): String = {
     conf.get(key, defaultValue)