osc: fix crashes related to field eventresponder being nil

I'm still getting some crashes after issue #3210 was fixed in commit
5beb230690. It's hard to reproduce those
crashes, they happen maybe once a month, so I guess it could be a race
condition. But in any case, I don't see anything wrong in applying some
defensive programming here.

For reference, here is what was happening on 0.23.0-1 from Debian
testing:

  Playing: ytdl://usL5CeP_row
   (+) Video --vid=1 (*) (h264)
    (+) Audio --aid=1 --alang=und (*) (aac)
    [osc]
    [osc] stack traceback:
    [osc]	  @osc.lua:2074: in function 'process_event'
    [osc]	  @osc.lua:2246: in function 'cb'
    [osc]	  mp.defaults:107: in function 'fn'
    [osc]	  mp.defaults:60: in function 'handler'
    [osc]	  mp.defaults:339: in function 'handler'
    [osc]	  mp.defaults:448: in function 'call_event_handlers'
    [osc]	  mp.defaults:485: in function 'dispatch_events'
    [osc]	  mp.defaults:441: in function <mp.defaults:440>
    [osc]	  [C]: in ?
    [osc]	  [C]: in ?
    [osc] Lua error: @osc.lua:2074: attempt to index field 'eventresponder' (a nil value)

Note that the location is different from where issue #3210 happens.

Signed-off-by: Roland Hieber <rohieb@rohieb.name>

player/lua/osc.lua

@@ -2072,6 +2072,7 @@ function process_event(source, what)
             if n == 0 then
                 --click on background (does not work)
             elseif n > 0 and not (n > #elements) and
+                not (elements[n].eventresponder == nil) and
                 not (elements[n].eventresponder[source .. "_" .. what] == nil) then
 
                 if mouse_hit(elements[n]) then