mmvideo: restore initial y value.

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

Bug introduced by libav
	commit a55d5bdc
	Date:   Tue Mar 6 15:15:42 2012 -0800

	    algmm: convert to bytestream2 API.

libavcodec/mmvideo.c

@@ -127,7 +127,7 @@ static int mm_decode_intra(MmContext * s, int half_horiz, int half_vert)
  */
 static int mm_decode_inter(MmContext * s, int half_horiz, int half_vert)
 {
-    int data_off = bytestream2_get_le16(&s->gb), y;
+    int data_off = bytestream2_get_le16(&s->gb), y = 0;
     GetByteContext data_ptr;
 
     if (bytestream2_get_bytes_left(&s->gb) < data_off)