Fix error "attempt to compare nil with number"

[ento]

This fixes an error that gets logged every time a video finishes playing:

...
[vd] Uninit decoder.
[client_osc]
[client_osc] stack traceback:
[client_osc]    mp.defaults:366: in function 'handler'
[client_osc]    mp.defaults:460: in function 'call_event_handlers'
[client_osc]    mp.defaults:494: in function 'dispatch_events'
[client_osc]    mp.defaults:453: in function <mp.defaults:452>
[client_osc]    [C]: at 0x5745da2c25d0
[client_osc]    [C]: at 0x5745da2c38d0
[client_osc] Lua error:
...jmrbar-mpv_thumbnail_script-0.4.2/scripts/client_osc.lua:1087:
attempt to compare nil with number
...

Tested on mpv version 0.29.0:

$ mpv --version
mpv 0.29.0-910-gd66eb93e5d Copyright © 2000-2019 mpv/MPlayer/mplayer2 projects
 built on Thu Oct 24 22:37:22 AKDT 2019
ffmpeg library versions:
   libavutil       56.35.101
   libavcodec      58.59.102
   libavformat     58.33.100
   libswscale      5.6.100
   libavfilter     7.65.100
   libswresample   3.6.100
ffmpeg version: N-95553-g155508c6e9

[w1kl4s]

Got broken for me with upgrade to mpv 0.30. This fixed the error.

➜  ~ mpv --version
mpv 0.30.0 Copyright © 2000-2019 mpv/MPlayer/mplayer2 projects
 built on UNKNOWN
ffmpeg library versions:
   libavutil       56.22.100
   libavcodec      58.35.100
   libavformat     58.20.100
   libswscale      5.3.100
   libavfilter     7.40.101
   libswresample   3.3.100
ffmpeg version: 4.1.4

[TheAMM]

There's now been several PRs for fixing this and I've sort of ignored them because I was surely going to rewrite this entire thing, but now I have to admit it's a very unknown ETA, so hey Let's Get This Merged.

[ento]

Oops, sorry for not looking at other PRs and thanks for the merge.

[garoto]

so hey Let's Get This Merged.

I recognize that reverse acronym.