The vsm_set vg pointer points to the last matched segment insertion, and
is used as an optimization when looking for existing entries in the
list. varnishd guarantees that insertions are always ordered, so there is
no need to search the preceeding entries on a successful match. When
parsing an index containg elements that included entries that are added
and then subsequently removed in a later entry, this pointer would be
advanced to the very end, failing to match the entries in between when
parsing the rest of the file.

Fix this mechanism by only updating the pointer on a successful match, and
also advancing it one step it if the entry it is pointing to is removed.

Note that these types of events are not supposed to be possible, because
varnishd will when rewriting the index only include the active ones,
removing any add-then-remove pairs. But if for whatever reason an attempt
is made to reread a list, with this patch it should not cause problems.