retry request: only access the HTTP data if in fact HTTP

When figuring out if the data stream needs to be rewound when the
request is to be resent, we must not access the HTTP struct unless the
protocol used is indeed HTTP...

Bug: http://curl.haxx.se/bug/view.cgi?id=3544688

lib/transfer.c

@@ -2060,7 +2060,9 @@ CURLcode Curl_retry_request(struct connectdata *conn,
                                 error just because nothing has been
                                 transferred! */
 
-    if(data->state.proto.http->writebytecount)
+
+    if((conn->handler->protocol&CURLPROTO_HTTP) &&
+       data->state.proto.http->writebytecount)
       return Curl_readrewind(conn);
   }
   return CURLE_OK;