netty: avoid unintentionally NullPointerException

[jyane]

grpc-java version: 1.8.0

I have encountered NullPointerException.

Http2Exception.streamError will raise an exception new StreamException(id, error, String.format(fmt, args), cause); but newStreamException passes a null value then raises a NullPointerException.

Http2Exception.streamError in netty is:
https://github.com/netty/netty/blob/33ddb83dc1e754039540906db50d76f06fa6819b/codec-http2/src/main/java/io/netty/handler/codec/http2/Http2Exception.java#L149

I think Throwable#getMessage is generally Nullable, so wrapping null value might be needed.
Is it intentionally? if it is I will close this PR.

Here is stacktrace:

java.lang.NullPointerException
	at java.util.regex.Matcher.getTextLength(Matcher.java:1283)
	at java.util.regex.Matcher.reset(Matcher.java:309)
	at java.util.regex.Matcher.<init>(Matcher.java:229)
	at java.util.regex.Pattern.matcher(Pattern.java:1093)
	at java.util.Formatter.parse(Formatter.java:2547)
	at java.util.Formatter.format(Formatter.java:2501)
	at java.util.Formatter.format(Formatter.java:2455)
	at java.lang.String.format(String.java:2940)
	at io.netty.handler.codec.http2.Http2Exception.streamError(Http2Exception.java:149)
	at io.grpc.netty.NettyServerHandler.newStreamException(NettyServerHandler.java:707)
	at io.grpc.netty.NettyServerHandler.onDataRead(NettyServerHandler.java:479)
	at io.grpc.netty.NettyServerHandler.access$700(NettyServerHandler.java:97)
	at io.grpc.netty.NettyServerHandler$FrameListener.onDataRead(NettyServerHandler.java:730)
	at io.netty.handler.codec.http2.DefaultHttp2ConnectionDecoder$FrameReadListener.onDataRead(DefaultHttp2ConnectionDecoder.java:240)
	at io.netty.handler.codec.http2.DefaultHttp2ConnectionDecoder$PrefaceFrameListener.onDataRead(DefaultHttp2ConnectionDecoder.java:591)
	at io.netty.handler.codec.http2.Http2InboundFrameLogger$1.onDataRead(Http2InboundFrameLogger.java:48)
	at io.netty.handler.codec.http2.DefaultHttp2FrameReader.readDataFrame(DefaultHttp2FrameReader.java:421)
	at io.netty.handler.codec.http2.DefaultHttp2FrameReader.processPayloadState(DefaultHttp2FrameReader.java:251)
	at io.netty.handler.codec.http2.DefaultHttp2FrameReader.readFrame(DefaultHttp2FrameReader.java:160)
	at io.netty.handler.codec.http2.Http2InboundFrameLogger.readFrame(Http2InboundFrameLogger.java:41)
	at io.netty.handler.codec.http2.DefaultHttp2ConnectionDecoder.decodeFrame(DefaultHttp2ConnectionDecoder.java:118)
	at io.netty.handler.codec.http2.Http2ConnectionHandler$FrameDecoder.decode(Http2ConnectionHandler.java:383)
	at io.netty.handler.codec.http2.Http2ConnectionHandler$PrefaceDecoder.decode(Http2ConnectionHandler.java:254)
	at io.netty.handler.codec.http2.Http2ConnectionHandler.decode(Http2ConnectionHandler.java:443)
	at io.netty.handler.codec.ByteToMessageDecoder.decodeRemovalReentryProtection(ByteToMessageDecoder.java:489)
	at io.netty.handler.codec.ByteToMessageDecoder.callDecode(ByteToMessageDecoder.java:428)
	at io.netty.handler.codec.ByteToMessageDecoder.channelRead(ByteToMessageDecoder.java:265)
	at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:362)
	at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:348)
	at io.netty.channel.AbstractChannelHandlerContext.fireChannelRead(AbstractChannelHandlerContext.java:340)
	at io.netty.channel.DefaultChannelPipeline$HeadContext.channelRead(DefaultChannelPipeline.java:1359)
	at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:362)
	at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:348)
	at io.netty.channel.DefaultChannelPipeline.fireChannelRead(DefaultChannelPipeline.java:935)
	at io.netty.channel.nio.AbstractNioByteChannel$NioByteUnsafe.read(AbstractNioByteChannel.java:134)
	at io.netty.channel.nio.NioEventLoop.processSelectedKey(NioEventLoop.java:645)
	at io.netty.channel.nio.NioEventLoop.processSelectedKeysOptimized(NioEventLoop.java:580)
	at io.netty.channel.nio.NioEventLoop.processSelectedKeys(NioEventLoop.java:497)
	at io.netty.channel.nio.NioEventLoop.run(NioEventLoop.java:459)
	at io.netty.util.concurrent.SingleThreadEventExecutor$5.run(SingleThreadEventExecutor.java:858)
	at io.netty.util.concurrent.DefaultThreadFactory$DefaultRunnableDecorator.run(DefaultThreadFactory.java:138)
	at java.lang.Thread.run(Thread.java:748)

[ejona86]

Yeah, that seems fair. I think the main alternative may actually be to have a hard-coded string, since the cause is maintained. But that'd take effort to verify, and this seems just as good.