Bug #109002 filename_to_tablename() handles temp table's name "p0#tmp" incorrectly

Description:
When running a REBUILD PARTITION p0 command on a paritioned table tr, the server will generate a temporary table named "p0#tmp" to store records from old partition. 

Look at the codes of function filename_to_tablename():
```
size_t filename_to_tablename(const char *from, char *to, size_t to_length,
                             bool stay_quiet, bool *has_errors) {
  uint errors;
  size_t res;
  DBUG_TRACE;
  DBUG_PRINT("enter", ("from '%s'", from));

  if (has_errors != nullptr) *has_errors = false;
  if (strlen(from) >= tmp_file_prefix_length &&
      !memcmp(from, tmp_file_prefix, tmp_file_prefix_length)) {
    /* Temporary table name. */
    res = (my_stpnmov(to, from, to_length) - to);
  } else {
    res = strconvert(&my_charset_filename, from, system_charset_info, to,
                     to_length, &errors);
    if (errors)  // Old 5.0 name
    {
      if (has_errors != nullptr) *has_errors = true;
      if (!stay_quiet) {
        LogErr(ERROR_LEVEL, ER_INVALID_OR_OLD_TABLE_OR_DB_NAME, from);
      }
      /*
        TODO: add a stored procedure for fix table and database names,
        and mention its name in error log.
      */
    }
  }

  DBUG_PRINT("exit", ("to '%s'", to));
  return res;
}
```

When the function filename_to_tablename() is called on the temporary table, it cannot recognize it as temporary table since the tmp_file_prefix is "#sql". What it does is trying converting the name using strconvert() with my_charset_filename, but it cannot deal with the character '#', so strconvert() returns with result "p0?tmp" and causes the caller wrongly printing an error ER_INVALID_OR_OLD_TABLE_OR_DB_NAME to the log.

How to repeat:
As above

Hi Mr. Hope Lee,

Thank you for your bug report.

We have analysed the function and we agree with you.

Verified as reported.