Backport to 10.0: MDEV-8779: mysqld got signal 11 in sql/opt_range_mrr.cc:100(step_down_to)

Post fix: initialize a variable

Author: spetrunia

sql/opt_range.cc

@@ -4921,7 +4921,7 @@ static bool create_partition_index_description(PART_PRUNE_PARAM *ppar)
  part_info->subpart_field_array;
  bool in_subpart_fields= FALSE;
  uint max_key_len= 0;
- uint cur_key_len;
+ uint cur_key_len= 0;
  for (uint part= 0; part < total_parts; part++, key_part++)
  {
  key_part->key= 0;