fix sign extension when decoding Backward Size#25
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Backward Size is an unsigned 32-bit field, but decodeStreamFooter ORs each byte into a long without widening, so a most-significant byte >= 0x80 makes the int shift sign-extend and yields a negative backwardSize. That value slips past the
backwardSize >= poscheck in SeekableXZInputStream and leads to an out-of-range seek. Widen the byte to long before shifting, like uncompSize in LZMAInputStream.