loader command interpreter should reset command_errmsg
The command interpreter does leave command_errmsg as is after printing its
content, assuming the next command will reset it in bf_command(). However,
in case the forth native word is defined as builtin, the bf_command is not
used and forth words will also end up the command_errmsg content printed.
Since command_errmsg is pointer to actual error message, which can be static
read only string, we can not just set *command_errmsg = '\0', instead we need
to reset the pointer itself.