Set invalid motion vector correctly

Motion vectors in tpl stats were set to 0 initially. In this CL, for
invalid motion vector, set it to INVALID_MV explicitly to ensure
correct motion vectors are used later.

Change-Id: I80caeb33326828579a92bf9252b4f7b93a93c2b7
diff --git a/av1/encoder/tpl_model.c b/av1/encoder/tpl_model.c
index e80645f..2f2e4d9 100644
--- a/av1/encoder/tpl_model.c
+++ b/av1/encoder/tpl_model.c
@@ -297,8 +297,10 @@
   best_mv.as_int = INVALID_MV;
 
   for (rf_idx = 0; rf_idx < INTER_REFS_PER_FRAME; ++rf_idx) {
-    if (ref_frame[rf_idx] == NULL) continue;
-    if (src_ref_frame[rf_idx] == NULL) continue;
+    if (ref_frame[rf_idx] == NULL || src_ref_frame[rf_idx] == NULL) {
+      tpl_stats->mv[rf_idx].as_int = INVALID_MV;
+      continue;
+    }
 
     const YV12_BUFFER_CONFIG *ref_frame_ptr = src_ref_frame[rf_idx];
     int ref_mb_offset =