Fix unexpected candidate disorder for '1'.

This is related to 64b45c5b7a43af88640fe51964020801bec6531c.
Originally, all symbols that do not have alternative symbol candidates would be assigned a '1' for `symbolKeyBuf`. And when trying to open a candidate window for these symbols, `HaninSymbolInput` would be called. However, `SymbolChoice` does not handle this situation correctly and causes some problems.  That is, a new symbol would be inserted without changing the original symbol.

The previous commit hides this problem, but when the candidate symbol is '1', this problem resurfaces.

This should close #44.