liblzma: Check the return value of lzma_index_append() in threaded encoder.

liblzma: Check the return value of lzma_index_append() in threaded encoder.

If lzma_index_append() failed (most likely memory allocation failure)
it could have gone unnoticed and the resulting .xz file would have
an incorrect Index. Decompressing such a file would produce the
correct uncompressed data but then an error would occur when
verifying the Index field.