Fix missing initialization in lzma_strm_init().

Fix missing initialization in lzma_strm_init().

With bad luck, lzma_code() could return LZMA_BUF_ERROR
when it shouldn't.

This has been here since the early days of liblzma.
It got triggered by the modifications made to the xz
tool in commit 18c10c30d2833f394cd7bce0e6a821044b15832f
but only when decompressing .lzma files. Somehow I managed
to miss testing that with Valgrind earlier.

This fixes <http://bugs.gentoo.org/show_bug.cgi?id=305591>.
Thanks to Rafał Mużyło for helping to debug it on IRC.