iov: avoid "orig_len may be used unitialized" warning

Signed-off-by: Wenchao Xia <xiawenc@linux.vnet.ibm.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
(cherry picked from commit 2be178a4)

Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>