If there are no arguments or invalid arguments given, swaynag will free
`swaynag.details.button_details` under the `cleanup` label in main. It
then called `swaynag_destroy`, which would attempt to free it again.
Since `swaynag.details.button_details` is either freed on line 106 of
main (when there is no detailed message) or added to `swaynag.buttons`
on line 103 of main, there is no reason to manually free it in
`swaynag_destroy`.
Although I cannot reproduce a double free on my system, for some reason,
it should have actually resulted in a double free in all code paths.
Brian Ashworth
eb527ac01a