After unit tests, clear port properties unconditionally

In MapOutputTrackerSuite, the "remote fetch" test sets spark.driver.port
and spark.hostPort, assuming that they will be cleared by
LocalSparkContext. However, the test never sets sc, so it remains null,
causing LocalSparkContext to skip clearing these properties. Subsequent
tests therefore fail with java.net.BindException: "Address already in
use".

This commit makes LocalSparkContext clear the properties even if sc is
null.

core/src/test/scala/org/apache/spark/LocalSparkContext.scala

@@ -40,17 +40,17 @@ trait LocalSparkContext extends BeforeAndAfterEach with BeforeAndAfterAll { self
   }
 
   def resetSparkContext() = {
-    if (sc != null) {
-      LocalSparkContext.stop(sc)
-      sc = null
-    }
+    LocalSparkContext.stop(sc)
+    sc = null
   }
 
 }
 
 object LocalSparkContext {
   def stop(sc: SparkContext) {
-    sc.stop()
+    if (sc != null) {
+      sc.stop()
+    }
     // To avoid Akka rebinding to the same port, since it doesn't unbind immediately on shutdown
     System.clearProperty("spark.driver.port")
     System.clearProperty("spark.hostPort")

core/src/test/scala/org/apache/spark/SharedSparkContext.scala

@@ -33,10 +33,8 @@ trait SharedSparkContext extends BeforeAndAfterAll { self: Suite =>
   }
 
   override def afterAll() {
-    if (_sc != null) {
-      LocalSparkContext.stop(_sc)
-      _sc = null
-    }
+    LocalSparkContext.stop(_sc)
+    _sc = null
     super.afterAll()
   }
 }