LevelPatternConverter.format may throw NPE

I found an issue when enabling asynchronous appenders with `-DLog4jContextSelector=org.apache.logging.log4j.core.async.AsyncLoggerContextSelector` and I also got a workaround to fix it.

Note that I invoke LogContext.setConfigLocation() then LogContext.reconfigure() in order to apply log4j2.xml programatically.

1. Stacktrace ... See the bottom of the Description as it is a large portion.
2. Workaround
Just to get rid of toString() at line 123 from the following snippet:

122 public void format(final LogEvent event, final StringBuilder output) {
123     output.append(levelMap == null ? event.getLevel().toString() : levelMap.get(event.getLevel()));
124 }

The workaround is as follows:

122 public void format(final LogEvent event, final StringBuilder output) {
123     output.append(levelMap == null ? event.getLevel() : levelMap.get(event.getLevel()));
124 }

3. Log4j2.xml

		<RollingRandomAccessFile name="MySizeRollingLog"
			fileName="foo.log"
			filePattern="foo-%d{MM-dd-yyyy}-%i.log.gz">
			<PatternLayout
				pattern="%d{yyyy-MM-dd HH:mm:ss,SSS z} %-5p [%t] (%F:%L) - [server($${sys:something})]-[%X{Something}]-%m%n" />
			<Policies>
				<TimeBasedTriggeringPolicy interval="1"
					modulate="true" />
				<SizeBasedTriggeringPolicy size="1000KB" />
			</Policies>
			<DefaultRolloverStrategy max="3" />
		</RollingRandomAccessFile>

I'm not sure why event.getLevel() was null but the workaround works for me.

Full stacktrace:
2013-12-05 09:36:58,947 ERROR An exception occurred processing Appender MySizeRollingLog java.lang.NullPointerException
at org.apache.logging.log4j.core.pattern.LevelPatternConverter.format(LevelPatternConverter.java:122)
at org.apache.logging.log4j.core.pattern.PatternFormatter.format(PatternFormatter.java:36)
at org.apache.logging.log4j.core.layout.PatternLayout.toSerializable(PatternLayout.java:167)
at org.apache.logging.log4j.core.layout.PatternLayout.toSerializable(PatternLayout.java:52)
at org.apache.logging.log4j.core.layout.AbstractStringLayout.toByteArray(AbstractStringLayout.java:45)
at org.apache.logging.log4j.core.appender.AbstractOutputStreamAppender.append(AbstractOutputStreamAppender.java:111)
at org.apache.logging.log4j.core.appender.RollingRandomAccessFileAppender.append(RollingRandomAccessFileAppender.java:96)
at org.apache.logging.log4j.core.config.AppenderControl.callAppender(AppenderControl.java:99)
at org.apache.logging.log4j.core.config.LoggerConfig.callAppenders(LoggerConfig.java:425)
at org.apache.logging.log4j.core.async.AsyncLoggerConfig.asyncCallAppenders(AsyncLoggerConfig.java:116)
at org.apache.logging.log4j.core.async.AsyncLoggerConfigHelper$Log4jEventWrapperHandler.onEvent(AsyncLoggerConfigHelper.java:218)
at org.apache.logging.log4j.core.async.AsyncLoggerConfigHelper$Log4jEventWrapperHandler.onEvent(AsyncLoggerConfigHelper.java:203)
at com.lmax.disruptor.BatchEventProcessor.run(BatchEventProcessor.java:133)
at java.util.concurrent.ThreadPoolExecutor$Worker.runTask(ThreadPoolExecutor.java:895)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:918)
at java.lang.Thread.run(Thread.java:695)