Although rB67e23b4b2967 turned the problem more recurrent, the warning
messages in the console always appear when `BKE_fluid_cache_free_all`
is called.
This is because of a bug in `BLI_filelist_dir_contents` as this function
calls `BLI_strdupcat` instead of `BLI_join_dirfile`
NOTE: Other places in Blender avoid this problem by making sure to add
a `SEP_STR` to the end of the directory.
Differential Revision: https://developer.blender.org/D16043
715c86d9e5