Fix for from names displayed in the attachment area when the message is a

Fix for from names displayed in the attachment area when the message is a
digest. After opening a digest message the rfc822_header object structure
which resulted we couldn't few the from name anymore when switched back to
the digest message.
We need to fix this later. See the comments.

git-svn-id: https://svn.code.sf.net/p/squirrelmail/code/trunk/squirrelmail@10703 7612ce4b-ef26-0410-bec9-ea0150e637f0