lzma: ignore unset filesizes

The Linux kernel build system changed how it compresses things with LZMA
such that the header no longer contains the filesize (it is instead set to
all F's).  So if we get a LZMA image that has -1 for the 64bit field,
let's just assume that the decompressed size is unknown and continue on.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>