fix: nil: Fix a latent NULL dereference in the DoH client request helper

isc__nm_http_request()'s error path reloaded sock->h2->connect.cstream after client_send() had already detached and freed it on a submit failure, dereferencing NULL. The helper is only used by the DoH unit tests. Guard the cleanup path against the detached stream.

Closes #6160

Merge branch '6160-fix-latent-NULL-dereference-in-http2' into 'main'

See merge request isc-projects/bind9!12247