Check chiSymbolBufLen in chewing_handle_Numlock()

When starting select mode, libchewing will copy chiSymbolCursor to
oldChiSymbolCursor. The oldChiSymbolCursor will copy back to chiSymbolCursor
after finishing select mode. This action assumes that chiSymbolBufLen never
changes during select mode, or chiSymbolCursor will out of bound and cause
crash eventually.

The chiSymbolBufLen will change when it exceeds limitation. Normally, it will
not happen during select mode because select mode does not increase
chiSymbolBufLen. However, if the chiSymbolBufLen already exceeds limitation
before entering select mode, it will change in select mode and libchewing might
crash after restoring chiSymbolBufLen to out of bound value.

This scenario happens when we use num lock to select symbol. Select symbol
increase chiSymbolBufLen and num lock does not check chiSymbolBufLen after
selecting. This cause chiSymbolBufLen out of bound and next select mode will
change the value of chiSymbolBufLen and cause crash.