Fix size_t temporary underflow in log_loaded_themes
Fix size_t temporary underflow in log_loaded_themes

`len` will underflow but will overflow right after, so it's not as bad as it
may appear. Still better not to under/overflow at all.

Fixes https://github.com/swaywm/sway/issues/3862

1ace/sway,SirCmpwn/sway,ascent12/sway,1ace/sway,ascent12/sway,ascent12/sway,1ace/sway