swab() on overlapping data undefined
Due to a recent bugfix in swab() I noticed that POSIX says about swab():
If copying takes place between objects that overlap, the behavior is undefined.
see https://pubs.opengroup.org/onlinepubs/9699919799/functions/swab.html xfwp uses swab() on overlapping src/dst: https://gitlab.freedesktop.org/xorg/app/xfwp/-/blob/master/io.c#L1043