Fix memory leak when attaching messages

Fix memory leak when attaching messages

Setting actx->idx[]->content->parts to NULL causes a memory leak,
because message attachments store a BODY there that needs to be freed.

I looked through the various use cases of ci_send_message() and
actually believe content->parts will be NULL for all other cases
except message attachments.  From the comment added to
delete_attachment():

  Other ci_send_message() message constructors are careful to free
  any body->parts, removing depth:
   - mutt_prepare_template() used by postponed, resent, and draft files
   - mutt_copy_body() used by the recvattach menu and $forward_attachments.

  I believe it is safe to completely remove the "content->parts =
  NULL" statement.  But for safety, am doing so only for the case
  it must be avoided: message attachments.

Co-authored-by: Austin Ray <austin@austinray.io>