select.lua: fix selecting the default subtitle line

Fixes 53796ca.

Author: guidocella

player/lua/select.lua

@@ -347,7 +347,7 @@ local function select_subtitle_line(secondary, keep_open)
         local duration = mp.get_property_native("duration", math.huge)
 
         for _, line in ipairs(lines) do
-            if not default_item and line.start <= time_pos then
+            if not keep_open and line.start <= time_pos then
                 default_item = #items + 1
             end