nghttp2: fix CVE-2023-35945

nghttp2: fix CVE-2023-35945

Envoy is a cloud-native high-performance edge/middle/service
proxy. Envoy’s HTTP/2 codec may leak a header map and
bookkeeping structures upon receiving `RST_STREAM` immediately
followed by the `GOAWAY` frames from an upstream server. In
nghttp2, cleanup of pending requests due to receipt of the
`GOAWAY` frame skips de-allocation of the bookkeeping structure
and pending compressed header. The error return [code path] is
taken if connection is already marked for not sending more
requests due to `GOAWAY` frame. The clean-up code is right after
the return statement, causing memory leak. Denial of service
through memory exhaustion. This vulnerability was patched in
versions(s) 1.26.3, 1.25.8, 1.24.9, 1.23.11.

References:
https://nvd.nist.gov/vuln/detail/CVE-2023-35945
https://github.com/envoyproxy/envoy/security/advisories/GHSA-jfxv-29pc-x22r

Signed-off-by: Yogita Urade <yogita.urade@windriver.com>
Signed-off-by: Steve Sakoman <steve@sakoman.com>