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.

Updates #37598

Change-Id: If0451e9c6431f6fab7137de43727297a80def05b
Reviewed-on: https://go-review.googlesource.com/c/go/+/221818
Reviewed-by: Brad Fitzpatrick <[email protected]>

Author: Bryan C. Mills

src/net/http/client.go

@@ -265,6 +265,12 @@ func send(ireq *Request, rt RoundTripper, deadline time.Time) (resp *Response, d
 		}
 		return nil, didTimeout, err
 	}
+	if resp == nil {
+		return nil, didTimeout, fmt.Errorf("http: RoundTripper implementation (%T) returned a nil *Response with a nil error", rt)
+	}
+	if resp.Body == nil {
+		return nil, didTimeout, fmt.Errorf("http: RoundTripper implementation (%T) returned a *Response with a nil Body", rt)
+	}
 	if !deadline.IsZero() {
 		resp.Body = &cancelTimerBody{
 			stop:          stopTimer,