Fix Emacs crash in mime-view

Fix Emacs crash in mime-view

Emacs crashes with segmentation fault when mime-view tries to display malformed
base64 attach like this one:

Content-Type: text/plain
Content-Transfer-Encoding: base64
Content-Disposition: attachment; filename="dashik.V4S"

Attachment is a binary file, but content-type is set to text/plain. Function
decode-coding-string crashes in such sutuation when encoding set to x-ctext.
Default encoding in APEL is x-unknown, but Wl reassigns it to x-ctext to make
summary buffer display non-ascii characters in message correctly. It is not
necessary to do for invalid attachments, so put default-mime-charset back to
x-unknown while displaying entities.