Fix unstable encoder behavior when row_tile is turned on

This commit fixes the use of uninitialized value in obmc motion
search, which causes unstable encoder behavior when row_tile is
not zero.

Change-Id: Ib6aa46ebedad90376ae2ac2edcae9e17f9234b6d

av1/common/onyxc_int.h

diff --git a/av1/common/onyxc_int.h b/av1/common/onyxc_int.h
index fd86ebd..0d42119 100644
--- a/av1/common/onyxc_int.h
+++ b/av1/common/onyxc_int.h
@@ -501,7 +501,7 @@
   xd->mb_to_right_edge = ((mi_cols - bw - mi_col) * MI_SIZE) * 8;
 
   // Are edges available for intra prediction?
-  xd->up_available = (mi_row != 0);
+  xd->up_available = (mi_row > tile->mi_row_start);
   xd->left_available = (mi_col > tile->mi_col_start);
   if (xd->up_available) {
     xd->above_mi = xd->mi[-xd->mi_stride];