Table export: ignoring invalid routes before marking them in export maps

Table export: ignoring invalid routes before marking them in export maps

Fast subsequent updates on filtered routes made the code crash because
no flags were set while ignoring them. And if these routes flapped, the
squashed export update crashed on a consistency check.

We ignore them unconditionally so we don't have to mark them at all and
we can convert them to NULL even before export maps are touched.