sidebar: fix %* index_format

 * mutt_FormatString() does not use SidebarWidth when calculate
   right part of the %*

 * mutt_wstr_trunc() does not reset mbrtowc() after error

 * mutt_wstr_trunc() compare signed and unsigned numbers to check for
   maxlen and maxwid limits and does not care about unprintable chars

Addresses: https://github.com/karelzak/mutt-kz/issues/58
Signed-off-by: Karel Zak <kzak@redhat.com>