Fix in update_valid_ref_frames_for_gm

In global motion, past reference frames w.r.t source_alt_ref_frame
have been updated correctly.

STATS_CHANGED

Change-Id: I83522ce3a8c39552b3a7258b84dabdbf81ed1f4b
diff --git a/av1/encoder/encodeframe.c b/av1/encoder/encodeframe.c
index bed7890..ac6a171 100644
--- a/av1/encoder/encodeframe.c
+++ b/av1/encoder/encodeframe.c
@@ -4893,7 +4893,7 @@
           order_hint_info, buf->display_order_hint,
           cm->cur_frame->display_order_hint);
       // Populate past and future ref frames
-      if (relative_frame_dist < 0) {
+      if (relative_frame_dist <= 0) {
         past_ref_frame[*num_past_ref_frames].distance =
             abs(relative_frame_dist);
         past_ref_frame[*num_past_ref_frames].frame = frame;