mutt: 1.8.0 -> 1.8.1

From the announce email:

    This is a bug fix release.  In particular, it has fixes for setenv,
    sidebar_whitelist, some refresh issues, and a potential segfault.
    Details can be seen in the ChangeLog file.

Signed-off-by: Matthias Beyer <mail@beyermatthias.de>