We're often iterating through messages while writing to another
buffer in our WWW interface, causing memory usage to multiply.
Since we know we won't need to keep the MIME object around in
some cases, and can tell msg_iter to clobber the on-stack
variable while it operates on subparts of multipart messages.
With xt/mem-msgview.t switched to multipart from the previous
commit, this shows a 13 MB memory reduction on that test.