btr_rec_copy_externally_stored_field() are safe.

row_merge_copy_blobs(): Note that the table is locked during index creation.
Therefore, none of its BLOBs can be freed.

row_sel_fetch_columns(): Note that rec must be protected by a page latch.
Add const qualifier to rec.

row_sel_get_clust_rec(): Note that the clustered index record is protected
by a page latch that was acquired when the persistent cursor was positioned
and that the latch will be freed by mini-transaction commit.

row_sel_try_search_shortcut(): Check the delete-mark flag before fetching
the columns.  Note that the clustered index record is protected
by a page latch that was acquired when the persistent cursor was positioned
and that the latch will be freed by mini-transaction commit.

row_sel(), row_search_for_mysql(): Note that the clustered index record
is protected by a page latch that was acquired when the persistent cursor
was positioned and that the latch will be freed by mini-transaction commit.

row_sel_field_store_in_mysql_format(): Add const qualifier to data.

row_sel_store_mysql_rec(), row_sel_push_cache_row_for_mysql():
Add const qualifier to rec.  Note that rec must be protected by a page latch.