libcangjie_cli segfaults on invalid input

$ libcangjie_cli 木,火
Segmentation fault (core dumped)
$ libcangjie_cli abcdef
Segmentation fault (core dumped)

The problem here is that the input is longer than 5 bytes, which is the maximum length of a Cangjie code.

libcangjie_cli needs to fail gracefully here, telling the user that these codes are invalid.