Align zero-length array to avoid unaligned accesses

[mattst88]

When make_tree() creates the left-justified base table (uint64_t *B)
it does so with 64-bit stores:

B[k] = sofar;

But B points to memory in the zero-length array "uint32_t tt[0]" at the
end of struct decoder_state. Since tt's type is uint32_t, it is only
aligned to a four byte boundary, and so the 64-bit store in make_tree()
causes an unaligned trap on strict platforms like sparc.

[mattst88]

If/when you take this patch, it would also be nice to make a 2.6 or 3.0 release. There is a ton of improvements since 2.5.