Add missing check in followTheOnlyLink() that leads to a panic

Bug introduced in PR #1290. Fixes #1631.

reader/scraper/scraper.go

@@ -118,11 +118,17 @@ func followTheOnlyLink(websiteURL, content string, rules, userAgent string, cook
 	if err != nil {
 		return "", err
 	}
+
+	if len(document.Find("body").Nodes) == 0 {
+		return content, nil
+	}
+
 	body := document.Find("body").Nodes[0]
 	if body.FirstChild.NextSibling != nil ||
 		body.FirstChild.Data != "a" {
 		return content, nil
 	}
+
 	// the body has only one child of <a>
 	var href string
 	for _, attr := range body.FirstChild.Attr {
@@ -131,16 +137,20 @@ func followTheOnlyLink(websiteURL, content string, rules, userAgent string, cook
 			break
 		}
 	}
+
 	if href == "" {
 		return content, nil
 	}
+
 	href, err = url.AbsoluteURL(websiteURL, href)
 	if err != nil {
 		return "", err
 	}
+
 	sameSite := url.Domain(websiteURL) == url.Domain(href)
 	if sameSite {
 		return fetchURL(href, rules, userAgent, cookie, allowSelfSignedCertificates, useProxy)
 	}
+
 	return fetchURL(href, rules, userAgent, "", false, false)
 }