Commits on Jun 21, 2021

1. compose: Check optipng is there before we use it …

   There are two places we can beef up the checks for optipng
   
     - When enabling its usage, make sure we found it first, and refuse to
       enable if we didn't.
     - In case `asc_compose_set_optipng_binary (NULL)` was called, check
       before spawning that we have *a* path. It's a programmer error to
       call `g_spawn_sync ()` with `NULL` in `argv[0]`, and the program
       will crash in that case. Also treat `NULL` as disabling optipng when
       passed to `asc_compose_set_optipng_binary`.
   
   In case the binary is removed mid-run or an invalid binary is passed to
   `asc_compose_set_optipng_binary`, the `g_spawn_sync` call itself will
   fail and we'll report a proper GError. This is the best way to handle
   it, as we avoid TOCTTOU races that way.

   Iain Lane committed Jun 21, 2021
   558b12b