Clear mailboxes list before its assignment.

On account change new account's mailboxes are being appended to the old
one's. This commit forces mailboxes list to be cleared before adding new
items to it.

autoconf/mutt_profile

@@ -16,3 +16,4 @@ set ssl_force_tls = yes
 
 bind index,pager g noop
 bind index gg first-entry
+unmailboxes *