Problem: using LOAD_FILE() in some cases we pass a file name string
without a trailing '\0' to fn_format() which relies on that however.
That may lead to valgrind warnings.

Fix: add a trailing '\0' to the file name passed to fn_format().


mysql-test/r/func_str.result:
  Fix for bug#44774: load_file function produces valgrind warnings
    - test result.
mysql-test/t/func_str.test:
  Fix for bug#44774: load_file function produces valgrind warnings
    - test case.
sql/item_strfunc.cc:
  Fix for bug#44774: load_file function produces valgrind warnings
    - passing a file name to fn_format(), file_name->c_ptr() replaced
      with file_name->c_ptr_safe() to ensure we have a trailing '\0'.