Directional intra prediction modes cause mismatch between encoder and decoder

[ycho]

Since one function, pred_directional(), is called by all those six directional prediction modes, the function is likely have a bug.

Try fix this issue at commit af9d4ab, because the commit after it seems to have another bug that also causes mismatch between encoder and decoder.

[shssoichiro]

Duplicate of #978, but I'm going to close that one because this one is more specific/accurate.