The tg_horz_boundary flag should always be 0 for the topmost
tile row, even when dependent-horztilegroups is enabled.
Otherwise, we end up trying to fetch data off the top of the
frame, which results in segfaults.

BUG=aomedia:1252

Change-Id: I7caaa2b38a21c05ffb13b6c72f41f8f6e1982b69