dump: Drop redundant check for empty dump

dump_init() first computes the size of the dump, taking the filter
area into account, and fails if its zero.  It then looks for memory in
the filter area, and fails if there is none.

This is redundant: if the size of the dump is zero, there is no
memory, and vice versa.  Delete this check.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20231031104531.3169721-6-armbru@redhat.com>