Fix SEGV with invalid appenders (#471)

If a non-appender class is attempted to be used as an appender, a SEGV can
occur.  Add explicit check to make sure that the class being used is in fact
an appender.

src/main/cpp/domconfigurator.cpp

@@ -224,6 +224,10 @@ AppenderPtr DOMConfigurator::parseAppender(Pool& p,
 	{
 		ObjectPtr instance = ObjectPtr(Loader::loadClass(className).newInstance());
 		AppenderPtr appender = LOG4CXX_NS::cast<Appender>(instance);
+		if(!appender){
+			LogLog::error(LOG4CXX_STR("Could not cast class of type [") + className + LOG4CXX_STR("] to appender"));
+			return AppenderPtr();
+		}
 		PropertySetter propSetter(appender);
 
 		appender->setName(subst(getAttribute(utf8Decoder, appenderElement, NAME_ATTR)));

src/test/cpp/xml/domtestcase.cpp

@@ -53,6 +53,7 @@ LOGUNIT_CLASS(DOMTestCase)
 #endif
 	LOGUNIT_TEST(test3);
 	LOGUNIT_TEST(test4);
+	LOGUNIT_TEST(invalidAppender);
 	LOGUNIT_TEST(invalidLevel);
 	LOGUNIT_TEST_SUITE_END();
 
@@ -228,6 +229,13 @@ LOGUNIT_CLASS(DOMTestCase)
 		LOGUNIT_ASSERT(exists);
 	}
 
+	void invalidAppender()
+	{
+		// Load an XML file that attempts to use a levelmatchfilter as an appender.
+		// We should not crash when loading this file.
+		DOMConfigurator::configure(LOG4CXX_TEST_STR("input/xml/DOMInvalidAppender.xml"));
+  }
+
 	void invalidLevel()
 	{
 		// Load an XML file that attempts to use a filter as a level.

src/test/resources/input/xml/DOMInvalidAppender.xml

@@ -0,0 +1,7 @@
+<log4j:configuration xmlns:log4j=' '>
+  <appender name="TEMP" class="levelmatchfilter"></appender>
+
+  <root>
+    <appender-ref ref="TEMP"/>
+  </root>
+</log4j:configuration>