This is a bug that was caught by a coverity run by Markus.  In
the error case when we errored out to exit_restore_open early in the
function, 'overlay_bs' was still NULL at that point, although it is
used to look up flags and perform a bdrv_reopen().

Move the overlay_bs lookup to where it is needed, and check for NULL
before restoring the flags.  Also get rid of the unneeded parameter
initialization.
Reported-By: NMarkus Armbruster <armbru@redhat.com>
Signed-off-by: NJeff Cody <jcody@redhat.com>
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>