mmvideo: restore initial y value.

This bug might have been exploitable (out of HEAP buffer writes)

Bug introduced by libav
	commit a55d5bdc6e28a2cfefc440d792de5cc4f02377e2
	Date:   Tue Mar 6 15:15:42 2012 -0800

	    algmm: convert to bytestream2 API.
