inserted, uncommitted clustered index records when determining if a
secondary index record that contains a column prefix of an externally
stored column is referencing the clustered index record.

field_ref_zero[]: A BLOB pointer full of zero, for use in comparisons.

btr_copy_externally_stored_field_prefix(): Assert that the BLOB pointer is set.

row_ext_lookup_ith(), row_ext_lookup(), row_ext_lookup_low(): Document
that field_ref_zero is returned when the BLOB cannot be fetched.

row_ext_lookup_low(): Return field_ref_zero and *len = 0 when the
BLOB pointer is unset.

row_build_index_entry(): Return NULL when a needed BLOB pointer cannot
be dereferenced (row_ext_lookup returns field_ref_zero).  Check the
return value for NULL in callers.

row_vers_impl_x_locked_off_kernel(): Avoid comparisons when
row_build_index_entry() returns NULL.

row_vers_old_has_index_entry(): Ignore records for which
row_build_index_entry() returns NULL.  The entry should never be NULL
in rollback, but it may be NULL in purge.

row_merge_buf_add(): Assert that row_ext_lookup() does not return
field_ref_zero.  The table will be locked during index creation.