vp9 non-rd pickmode: Fix logic in reference masking.

This change makes sure last reference with zero mv
is always checked for mode selection.

No change in metrics.

Change-Id: Iaf01877bf34272b966c78bfe18daad882a0a419e

vp9/encoder/vp9_pickmode.c

diff --git a/vp9/encoder/vp9_pickmode.c b/vp9/encoder/vp9_pickmode.c
index 9db044f..8aafae1 100644
--- a/vp9/encoder/vp9_pickmode.c
+++ b/vp9/encoder/vp9_pickmode.c
@@ -1252,7 +1252,8 @@
     if (const_motion[ref_frame] && this_mode == NEARMV)
       continue;
 
-    if (!(this_mode == ZEROMV && ref_frame == LAST_FRAME)) {
+    if (!(frame_mv[this_mode][ref_frame].as_int == 0 &&
+        ref_frame == LAST_FRAME)) {
       i = (ref_frame == LAST_FRAME) ? GOLDEN_FRAME : LAST_FRAME;
       if ((cpi->ref_frame_flags & flag_list[i]) && sf->reference_masking)
         if (x->pred_mv_sad[ref_frame] > (x->pred_mv_sad[i] << 1))