vinyl: do not panic if secondary index is inconsistent with primary

Although the bug in vy_task_dump_complete() due to which a tuple could
be lost during dump was fixed, there still may be affected deployments
as the bug was persisted on disk. To avoid occasional crashes on such
deployments, let's make vinyl_iterator_secondary_next() skip tuples that
are present in a secondary index but missing in the primary.

Closes #3393