Fix segfault when viewing text attachments in compose menu. (closes #3644)

Fix segfault when viewing text attachments in compose menu.  (closes #3644)

The segfault was introduced in changeset b9f9e3147eb4.  Since decoding
and charset conversion aren't needed for attachments when composing a
message, this patch reverts to just using mutt_save_attachment() to view
"raw data" for text attachments in the compose/send case.

This patch is based on Michael Elkins' patch at
http://dev.mutt.org/trac/attachment/ticket/3644/view_attach_compose_segfault
with just a missing return value check added.