fix the bugs introduced in r1591.

row_rec_to_index_entry_low(): Clear "n_ext".  Do not allow it to be NULL.
Add const qualifier to dict_index_t*.

row_rec_to_index_entry(): Add the parameters "offsets" and "n_ext".

btr_cur_optimistic_update(): Add an assertion that there are no externally
stored columns.  Remove the unreachable call to btr_cur_unmark_extern_fields()
and the preceding unnecessary call to rec_get_offsets().

btr_push_update_extern_fields(): Remove the parameters index, offsets.
Only report the additional externally stored columns of the update vector.

row_build(), trx_undo_rec_get_partial_row(): Flag externally stored columns
also with dfield_set_ext().

rec_copy_prefix_to_dtuple(): Assert that there are no externally stored
columns in the prefix.

row_build_row_ref(): Note and assert that the index is a secondary index,
and assert that there are no externally stored columns.

row_build_row_ref_fast(): Assert that there are no externally stored columns.

rec_offs_get_n_alloc(): Expose the function.

row_build_row_ref_in_tuple(): Assert that there are no externally stored
columns in a record of a secondary index.

row_build_row_ref_from_row(): Assert that there are no externally stored
columns.

row_upd_check_references_constraints(): Add the parameter offsets, to
avoid a redundant call to rec_get_offsets().

row_upd_del_mark_clust_rec(): Add the parameter offsets.  Remove
duplicated code.

row_ins_index_entry_set_vals(): Copy the external storage flag.

sel_pop_prefetched_row(): Assert that there are no externally stored
columns.

row_scan_and_check_index(): Copy offsets to a temporary heap across
the invocation of row_rec_to_index_entry().