pci: move dereferencing of root only after verifying valid root pointer

Signed-off-by: Saravanakumar <saravanakumar.punith@gmail.com>
Reviewed-by: Andreas Färber <afaerber@suse.de>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
