net/http: verify RoundTripper invariants in the send function
Issue #37598 reports a nil-panic in *Client.send that can
only occur if one of the RoundTripper invariants is violated.
Unfortunately, that condition is currently difficult to diagnose: it
manifests as a panic during a Response field access, rather than
something the user can easily associate with an specific erroneous
RoundTripper implementation.
No test because the new code paths are supposed to be unreachable.