fix #715 prevent NullPointerException with bootstrap classloader

Author: Nowheresly

logback-classic/src/main/java/ch/qos/logback/classic/util/ContextInitializer.java

@@ -100,6 +100,10 @@ public void autoConfig(ClassLoader classLoader) throws JoranException {
     }
 
     private Configurator instantiateConfiguratorByClassName(String configuratorClassName, ClassLoader classLoader) {
+        if (classLoader == null) {
+            contextAware.addInfo("Instantiation failure: null ClassLoader");
+            return null;
+        }
         try {
             Class<?> classObj = classLoader.loadClass(configuratorClassName);
             return (Configurator) classObj.getConstructor().newInstance();

logback-classic/src/test/java/ch/qos/logback/classic/util/ContextInitializerTest.java

@@ -122,6 +122,13 @@ public void autoConfigFromServiceLoaderJDK6andAbove() throws Exception {
         assertSame(loggerContext, MockConfigurator.context);
     }
 
+    @Test
+    public void autoConfigFromBootstrapClassLoader() throws Exception {
+        assertNull(MockConfigurator.context);
+        new ContextInitializer(loggerContext).autoConfig(null);
+        assertNull(MockConfigurator.context);
+    }
+
     @Test
     public void autoConfigFromServiceLoaderJDK5() throws Exception {
         assumeTrue(isJDK5());