UncompressedLZMA2OutputStream: Fix handling of the finished state.

The variable "finished" was set to "true" in a wrong method
(compare to LZMA2OutputStream) and incorrectly wouldn't
get set in close(). This can lead to wrong behavior if
a buggy application calls functions other than close()
after close() has been called. Non-buggy application don't
do such things and aren't affected by this bug.