http2: do no throw in writeHead if state.closed

The http1 implementation does not throw if the connection is down. The http2 compat implementation should do the same.

See: https://github.com/fastify/fastify-http-proxy/issues/51.

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• tests and/or benchmarks are included
• commit message follows commit guidelines