avformat/http: restore offset on http_open_cnx() failure

The retry path restores this offset, but the failure path does not. This
is especially important for the case of the continuation handler in
http_read_stream(), which may result in subsequent loop iterations (after
repeated failures to read additional data) seeking to the wrong offset.

libavformat/http.c

@@ -512,6 +512,7 @@ redo:
     return 0;
 
 fail:
+    s->off = off;
     if (s->hd)
         ffurl_closep(&s->hd);
     if (ret < 0)