Update SparkJoinsTest.java

Author: helenahm

spark/src/test/java/com/matthewrathbone/sparktest/SparkJoinsTest.java

@@ -37,8 +37,9 @@ public void setUp() {
 
 	  @After
 	  public void tearDown() {
-	    sc.stop();
-	    sc = null;
+	    if (sc != null){
+            	sc.stop();
+	    }
 	  }
 
 	  @Test