externally stored columns.

innodb-zip.test: Correct the test case.  Without the fixes, the test
would fail, because the BLOB would be prepended with a 768-byte prefix
of the data.

row_upd_index_replace_new_col_vals_index_pos(),
row_upd_index_replace_new_col_vals(): Use only one "heap"
parameter that must be non-NULL.  When fetching externally
stored columns, use upd_field_t::orig_len.

upd_get_field_by_field_no(): New accessor function, for retrieving
an field from an update vector by field_no.

row_upd_index_replace_new_col_val(): New function, for replacing the
value from an update vector.  This used to be duplicated code in
row_upd_index_replace_new_col_vals_index_pos() and
row_upd_index_replace_new_col_vals().