libbacktrace: don't skip initial aligned byte in uncompressed block

Patch from Rui Ueyama, who says:

libbacktrace occasionally fails to decompress compressed debug info
even though the sections contain valid zlib streams. The cause of the
issue is an off-by-one error.

If a zlib data block is a plain data (uncompressed data), the next two
bytes contain the size of the block. These two bytes value is byte-
aligned, so if we read-ahead more than 8 bits, we need to unread it.

So, the correct condition to determine whether or not we need to
unread a byte is bits >= 8 and not bits > 8. Due to this error,
if the last read bits happened to end at a byte boundary, the next
byte would be skipped. That caused the decompression failure.

This bug was originally reported against the mold linker.
rui314/mold#402

	* elf.c (elf_zlib_inflate): Don't skip initial aligned byte in
	uncompressed block.