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This problem is similar to MDEV-10306.

1. Fixing Item_str_conv::val_str(String *str) to return the result in "str",
   and to use tmp_value only as a temporary buffer for args[0]->val_str().
   The new code version now guarantees that the result is always returned in
   "str". The trick with copy_if_not_alloced() is not used any more.

2. The change #1 revealed the same problem in SUBSTRING_INDEX(),
   so some tests with combinations of UPPER()/LOWER() and SUBSTRING_INDEX()
   started to fail. Fixing Item_func_substr_index::val_str() the same way,
   to return the result in "str" and use tmp_value as a temporary buffer
   for args[0]->val_str().