Fix possible bugs in HTTP/2 Codec

[hyperxpro]

Motivation:

1. data can be null
   

   netty/codec-http2/src/main/java/io/netty/handler/codec/http2/DefaultHttp2FrameWriter.java

   Line 224 in 69b44c6

   ByteBuf lastFrame = data.readSlice(frameDataBytes);

2. i >= current.length is always true
   

   netty/codec-http2/src/main/java/io/netty/handler/codec/http2/ReadOnlyHttp2Headers.java

   Line 831 in 69b44c6

   if (i >= current.length && current == pseudoHeaders) {

3. newValue != 0 is always true
   

   netty/codec-http2/src/main/java/io/netty/handler/codec/http2/AbstractHttp2StreamChannel.java

   Line 261 in 69b44c6

   if (oldValue == 0 && newValue != 0) {

4. (parentStream == null) is already handled by shouldIgnoreHeadersOrDataFrame(ctx, streamId, parentStream, "PUSH_PROMISE") because it'll throw an error.
   

   netty/codec-http2/src/main/java/io/netty/handler/codec/http2/DefaultHttp2ConnectionDecoder.java

   Line 510 in 69b44c6

   if (parentStream == null) {

   
   

   netty/codec-http2/src/main/java/io/netty/handler/codec/http2/DefaultHttp2ConnectionDecoder.java

   Line 582 in 69b44c6

   if (stream == null) {

5. for (int i = 0; i < name.length(); ++i) { name can be null because we don't return in null check.
   

   netty/codec-http2/src/main/java/io/netty/handler/codec/http2/DefaultHttp2Headers.java

   Line 65 in 69b44c6

   for (int i = 0; i < name.length(); ++i) {

   
   

   netty/codec-http2/src/main/java/io/netty/handler/codec/http2/DefaultHttp2Headers.java

   Line 43 in 69b44c6

   if (name == null || name.length() == 0) {

6. hpackHeaderFields[cursor++]; can be null and it'll throw NPE when we try to access it.
   

   netty/codec-http2/src/main/java/io/netty/handler/codec/http2/HpackDynamicTable.java

   Line 188 in 69b44c6

   HpackHeaderField entry = hpackHeaderFields[cursor++];

7. next = current[i + 1]; can lead to Array Index Out of Bound error.
   

   netty/codec-http2/src/main/java/io/netty/handler/codec/http2/ReadOnlyHttp2Headers.java

   Line 826 in 69b44c6

   next = current[i + 1];

Modification:
Fixed all the above possible bugs.

Result:
Better code

[netty-bot]

Can one of the admins verify this patch?

[hyperxpro]

HTTP/2 Codec tests passing perfectly.

[hyperxpro]

@chrisvest PTAL

[normanmaurer]

@netty-bot test this please

[chrisvest]

@netty-bot test this please

[hyperxpro]

Ready to merge if code looks fine to everyone.

[artem-smotrakov]

@hyperxpro I see you fix some issues reported by LGTM. Would you mind suppressing the warnings that appeared to be false-positives?

https://lgtm.com/help/lgtm/alert-suppression#java

[hyperxpro]

@artem-smotrakov I don't think there is any false positive in this.