Fix buf overflow when doing temporal filtering for fwd-kf.

Change-Id: Ib9c07d3e889d012c3c1d34cba3e3466d27ae1083
diff --git a/av1/encoder/temporal_filter.c b/av1/encoder/temporal_filter.c
index 62a4d3b..2483508 100644
--- a/av1/encoder/temporal_filter.c
+++ b/av1/encoder/temporal_filter.c
@@ -1040,7 +1040,7 @@
   num_frames = AOMMIN(num_frames + adjust_num, lookahead_depth);
 
   if (frame_type == KEY_FRAME) {
-    num_before = is_forward_keyframe ? num_frames / 2 : 0;
+    num_before = AOMMIN(is_forward_keyframe ? num_frames / 2 : 0, max_before);
     num_after = AOMMIN(num_frames - 1, max_after);
   } else {
     int gfu_boost = av1_calc_arf_boost(&cpi->ppi->twopass, &cpi->twopass_frame,