vinyl: fix EQ check in run iterator

vy_run_iterator_seek() is supposed to check that the resulting statement
matches the search key in case of ITER_EQ, but if the search key lies at
the beginning of the slice, it doesn't. As a result, vy_point_lookup()
may fail to find an existing tuple as demonstrated below.

Suppose we are looking for key {10} in the primary index which consists
of an empty mem and two runs:

    run 1: DELETE{15}
    run 2: INSERT{10}

vy_run_iterator_next() returns DELETE{15} for run 1 because of the
missing EQ check and vy_point_lookup() stops at run 1 (since the
terminal statement is found) and mistakenly returns NULL.

The issue manifests itself as crash in vinyl_iterator_secondary_next(),
when we fail to find the tuple in the primary index corresponding to a
statement found in a secondary index.

Part of #3393