liblzma: Fix a memory leak in error path of lzma_index_dup().

lzma_index_dup() calls index_dup_stream() which, in case of
an error, calls index_stream_end() to free memory allocated
by index_stream_init(). However, it illogically didn't
actually free the memory. To make it logical, the tree
handling code was modified a bit in addition to changing
index_stream_end().

Thanks to Evan Nemerson for the bug report.