ytdl_hook: fix crash on missing track bitrate

Some tracks happen to lack bitrate information (ie. no tbr value).
In that case, just ignore the track while computing the max bitrate.

For an example, this is a stream in which all audio tracks
have no bitrate: https://www.raiplay.it/dirette/rai1

player/lua/ytdl_hook.lua

@@ -507,7 +507,7 @@ local function add_single_video(json)
 
         if requested_formats then
             for _, track in pairs(requested_formats) do
-                max_bitrate = track.tbr > max_bitrate and
+                max_bitrate = (track.tbr and track.tbr > max_bitrate) and
                     track.tbr or max_bitrate
             end
         elseif json.tbr then