Comment 4 for bug 1515591

Another suspicious "memcmp()" is in the "get_constant_key_infix()" function
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    if (cur_range->maybe_null &&
         cur_range->min_value[0] && cur_range->max_value[0])
    {
      /*
        cur_range specifies 'IS NULL'. In this case the argument points
        to a "null value" (a copy of is_null_string) that we do not
        memcmp(), or memcpy to a field.
      */
      DBUG_ASSERT (field_length > 0);
      *key_ptr= 1;
      key_ptr+= field_length;
      *key_infix_len+= field_length;
    }
    else if (memcmp(cur_range->min_value, cur_range->max_value, field_length) == 0)
    { /* cur_range specifies an equality condition. */
      memcpy(key_ptr, cur_range->min_value, field_length);
      key_ptr+= field_length;
      *key_infix_len+= field_length;
    }
    else
      return false;
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