When partition pruning is used we have to initialize key_part->flag in the 
create_partition_index_descr() as it's checked in the get_mm_leaf().


sql/opt_range.cc:
  Fix for bug #23762: partition.test fails (partition_hash, partition_pruning as well)
    - set key_part->flag to 0.
sql/opt_range.h:
  Fix for bug #23762: partition.test fails (partition_hash, partition_pruning as well)
    - comment added.