Do not store user phrase when its length is 1

chewing_handle_Enter() uses chewing_handle_CtrlNum() to store user phrase.
However, instead of storing user phrase, chewing_handle_CtrlNum() turns on
select window when key is 0 or 1. So Enter key might accidently turns on
select window.

The problem is, chewing_handle_Enter() modifies chiSymbolCursor before
calling chewing_handle_CtrlNum(), while chewing_handle_CtrlNum() needs
correct chiSymbolCursor to turn on select window. In this case,
chiSymbolCursor might set to an invalid value and crash the library
eventually.

To prevent this, chewing_handle_Enter() will not call
chewing_handle_CtrlNum() when length of user phrase is less then 2.