columns again

This is follow-up to Bug #54358. Not all occurrences of the bug were fixed.
We need to check all calls to btr_copy_externally_stored_field_prefix_low()
and do the right thing when the pointer to the off-page column is null
(full of zero bytes).

It turns out that only the call to btr_copy_externally_stored_field_prefix()
in row_sel_sec_rec_is_for_blob() needs to be changed.

For fetching complete off-page columns rather than prefixes, the function
btr_rec_copy_externally_stored_field() already checks if the pointer
is null (all-zero).  Two of its callers (row_merge_copy_blobs() and
row_sel_fetch_columns()) are never executed as READ COMMITTED and can
rightfully assert that the fetch succeeded.  The third caller,
row_sel_store_mysql_rec(), already does the right thing.

The calls in row_upd_ext_fetch() and trx_undo_page_fetch_ext() must
expect that the off-page column exists. Update and rollback are
locking operations, never READ UNCOMMITTED.