fix: WINDOW_UPDATE 0-increment RFC conformance

[ArniDagur]

Previously, h2 emitted a connection level GOAWAY(PROTOCOL_ERROR) when a peer sent a WINDOW_UPDATE frame with an increment of zero, regardless of which stream the bad frame was on. This killed every stream on the connection. Instead, we should only reset that particular stream.

This is described in https://www.rfc-editor.org/rfc/rfc9113.html#section-6.9-9

See:

• https://github.com/golang/net/blob/89624e152475881f05bb88aca261239d1b45a833/http2/frame.go#L1015-L1020

[seanmonstar]

Do you mind mentioning what you hope to unlock? From the looks of the code, it will already return an error, right? Just that it's a connection error instead of stream?

[ArniDagur]

This came from an audit of h2 for bugs, by comparing against Golang / nghttp2. This is not a big issue as clients shouldn't be sending these zero increment updates, but it's technically RFC non-conformant, and also kills all streams rather than just the specific one.