bug#44766: valgrind error when using convert() in a subquery

Problem: input and output buffers may be the same 
converting a string to some charset. 
That may lead to wrong results/valgrind warnings.  

Fix: use different buffers.


mysql-test/r/cast.result:
  Fix for bug#44743: Join in combination with concat does not always work
          bug#44766: valgrind error when using convert() in a subquery
    - test result.
mysql-test/r/func_concat.result:
  Fix for bug#44743: Join in combination with concat does not always work
          bug#44766: valgrind error when using convert() in a subquery
    - test result.
mysql-test/t/cast.test:
  Fix for bug#44743: Join in combination with concat does not always work
          bug#44766: valgrind error when using convert() in a subquery
    - test case.
mysql-test/t/func_concat.test:
  Fix for bug#44743: Join in combination with concat does not always work
          bug#44766: valgrind error when using convert() in a subquery
    - test case.
sql/item.cc:
  Fix for bug#44743: Join in combination with concat does not always work
          bug#44766: valgrind error when using convert() in a subquery
    - comment added.
sql/item_strfunc.cc:
  Fix for bug#44743: Join in combination with concat does not always work
          bug#44766: valgrind error when using convert() in a subquery
    - '&args[0]->str_value' used as a parameter of args[0]->val_str(),
      as 'str' may be equal to 'str_value' which we use as the output buffer
      converting strings.
sql/sql_string.cc:
  Fix for bug#44743: Join in combination with concat does not always work
          bug#44766: valgrind error when using convert() in a subquery
    - input and output buffers must NOT be the same.