column of a compressed table, the BTR_EXTERN_LEN field in the BLOB pointer
will be written as 0.  Tolerate this in the functions that deal with
externally stored columns.  This fixes Issue #80 and was posted at rb://26.

Note that the clustered index record is always deleted or purged last,
after any secondary index records referring to it have been deleted.

btr_free_externally_stored_field(): On an uncompressed table, zero out
the BTR_EXTERN_LEN, so that half-deleted BLOBs can be detected after
crash recovery.

btr_copy_externally_stored_field_prefix(): Return 0 if the BLOB has been
half-deleted.

row_upd_ext_fetch(): Assert that the externally stored column exists.

row_ext_cache_fill(): Allow btr_copy_externally_stored_field_prefix()
to return 0.

row_sel_sec_rec_is_for_blob(): Return FALSE if the BLOB has been half-deleted.
This is correct, because the clustered index record would have been deleted
or purged last, after any secondary index records referring to it had been
deleted.