Reduce the MFMV_STACK_SIZE value

Drop it from 4 to 3 to reflect the actual use case.

Change-Id: Ifdadaf053153c21b4b4fef40a3298a557fd2ef92

av1/common/mvref_common.c

diff --git a/av1/common/mvref_common.c b/av1/common/mvref_common.c
index c1bc4ce..1cdefb5 100644
--- a/av1/common/mvref_common.c
+++ b/av1/common/mvref_common.c
@@ -1659,7 +1659,7 @@
 
   if (alt_frame_index < cur_frame_index) return;
 
-  int ref_stamp = 2;
+  int ref_stamp = MFMV_STACK_SIZE - 1;
 
   // ======================
   // Process last frame

av1/common/onyxc_int.h

diff --git a/av1/common/onyxc_int.h b/av1/common/onyxc_int.h
index 2248178..353e45f 100644
--- a/av1/common/onyxc_int.h
+++ b/av1/common/onyxc_int.h
@@ -105,7 +105,7 @@
 } REFRESH_FRAME_CONTEXT_MODE;
 
 #if CONFIG_MFMV
-#define MFMV_STACK_SIZE 4
+#define MFMV_STACK_SIZE 3
 
 typedef struct {
   int_mv mfmv[INTER_REFS_PER_FRAME][MFMV_STACK_SIZE];