net/http: don't crash in Request.WithContext if Request.URL is nil

Fixes #20601

Change-Id: I296d50dc5210a735a2a65d64bfef05d14c93057b
Reviewed-on: https://go-review.googlesource.com/45073
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Rhys Hiltner <rhys@justin.tv>
Reviewed-by: Emmanuel Odeke <emm.odeke@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/net/http/request.go

@@ -333,9 +333,11 @@ func (r *Request) WithContext(ctx context.Context) *Request {
 	// Deep copy the URL because it isn't
 	// a map and the URL is mutable by users
 	// of WithContext.
-	r2URL := new(url.URL)
-	*r2URL = *r.URL
-	r2.URL = r2URL
+	if r.URL != nil {
+		r2URL := new(url.URL)
+		*r2URL = *r.URL
+		r2.URL = r2URL
+	}
 
 	return r2
 }

src/net/http/request_test.go

@@ -799,6 +799,13 @@ func TestWithContextDeepCopiesURL(t *testing.T) {
 	if firstURL == secondURL {
 		t.Errorf("unexpected change to original request's URL")
 	}
+
+	// And also check we don't crash on nil (Issue 20601)
+	req.URL = nil
+	reqCopy = req.WithContext(context.Background())
+	if reqCopy.URL != nil {
+		t.Error("expected nil URL in cloned request")
+	}
 }
 
 // verify that NewRequest sets Request.GetBody and that it works