liblzma: Don't create an empty Block in lzma_stream_buffer_encode().
liblzma: Don't create an empty Block in lzma_stream_buffer_encode().

Empty Block was created if the input buffer was empty.
Empty Block wastes a few bytes of space, but more importantly
it triggers a bug in XZ Utils 5.0.1 and older when trying
to decompress such a file. 5.0.1 and older consider such
files to be corrupt. I thought that no encoder creates empty
Blocks when releasing 5.0.2 but I was wrong.

pombreda/go-liblzma,pombreda/go-liblzma,pombreda/go-liblzma