Fix 'j', 'k' in select mode cannot handle symbol selecting

The original code uses ChoiceFirstAvail() when using 'j', 'k' in select mode.
However, ChoiceFirstAvail() can only handle word and phrase selectin. If
encounting symbol selecting, ChoiceFirstAvail() will fail to setup the
selecting, and lib will crash eventually. See [1].

[1] http://code.google.com/p/chewing/issues/detail?id=472