Curl_retry_request: check return code!

Curl_readrewind() was called without checking its return code, which
could lead to badness.

Bug: http://curl.haxx.se/bug/view.cgi?id=3349227

lib/transfer.c

@@ -2081,7 +2081,7 @@ CURLcode Curl_retry_request(struct connectdata *conn,
                                 transferred! */
 
     if(data->state.proto.http->writebytecount)
-      Curl_readrewind(conn);
+      return Curl_readrewind(conn);
   }
   return CURLE_OK;
 }