Optimized loop bounds to allow the compiler to unroll the loop.

This has no measurable impact on large files but improves small file
decompression by ~1-2% for 10kB, benchmarked with:

head -c 10000 silesia.tar > /tmp/test
make CC=/usr/local/bin/clang-9 BUILD_STATIC=1 && ./lzbench -ezstd -t1,5 /tmp/test