Introduced by the fix for bug#44766.

Problem: it's not correct to use args[0]->str_value as a buffer,
because args[0] may need this buffer for its own purposes.

Fix: adding a new class member tmp_value to use as return value.

  @ mysql-test/r/ctype_many.result
  @ mysql-test/t/ctype_many.test
  Adding tests

  @ sql/item_strfunc.cc
  Changing code into traditional style:
  use "str" as a buffer for the argument and tmp_value for the result value.

  @ sql/item_strfunc.h
  Adding tmp_value