Fix for NPE

Author: MarcMil

soot-infoflow/src/soot/jimple/infoflow/util/SystemClassHandler.java

@@ -53,6 +53,8 @@ public static void setInstance(SystemClassHandler instance) {
 	 * @return True if the given class belongs to a system package, otherwise false
 	 */
 	public boolean isClassInSystemPackage(SootClass clazz) {
+		if (clazz == null)
+			return false;
 		FlowDroidSystemClassChecked checked = (FlowDroidSystemClassChecked) clazz
 				.getTag(FlowDroidSystemClassChecked.TAG_NAME);
 		boolean r;