Deobfuscate ff_huff_build_tree.

I have no idea what the idea was behind the original code,
but the new code is equivalent to it.
In that loop that places the new node nodes[j] contains
always the data of the new node (since the steps are always
in order: FFSWAP copies node[j] to node[j-1], j is decremented).
Thus nodes[j].no == i and nodes[j].sym == HNODE.
make fate still passes and contains VP6 samples which use
FF_HUFFMAN_FLAG_HNODE_FIRST.

Signed-off-by: Reimar Döffinger <Reimar.Doeffinger@gmx.de>
