When working on MWL#247 I forgot to adjust the function create_hj_key_for_table()
that created a key definition for hash join keys. The modified function must
set the values of the fields ext_key_parts, ext_key_flags, ext_key_part_map
added to the key definition structure in MWL#247.
The fields ext_key_flags and ext_key_part_map must be initialized for any
key, even for a MyISAM key that never is considered by the optimizer as one
extended by hidden components.
The patch for MWL #247 forgot to initialize the TABLE::ext_key_parts and
TABLE::ext_key_flags of the temporary tables by a query. This could cause
crashes for queries the execution of which needed creation of temporary
tables.
