Avoid encoding garbage when ivfenc encounters an unsupported Y4M file.

Avoid encoding garbage when ivfenc encounters an unsupported Y4M file.

This change stops ivfenc from treating unsupported Y4M files as raw
input.

For example, if given an interlaced Y4M file, ivfenc treated the input
as if it were raw data because the unsupported Y4M file case previously
fell through without being handled.

Change-Id: I06caa50f3448e6388741a77346daaebf77c277e1