Reset cm->next_ref_frame_map to NULL instead of -1

This follows commit e7ebf904b0e763997d23b9a17ea98e5573843e76, which
changed buffer references from indices to pointers. So an invalid
index of -1 should be replaced by a NULL pointer.

BUG=aomedia:2244

Change-Id: I0c07e9e6f2ec0c7d0a1019947d33b17ab50e38e9

av1/decoder/decodeframe.c

diff --git a/av1/decoder/decodeframe.c b/av1/decoder/decodeframe.c
index 54d3a97..0811c05 100644
--- a/av1/decoder/decodeframe.c
+++ b/av1/decoder/decodeframe.c
@@ -4766,8 +4766,10 @@
   int i;
 
   // We have not stored any references to frame buffers in
-  // cm->next_ref_frame_map, so we can directly reset it to all -1.
-  memset(&cm->next_ref_frame_map, -1, sizeof(cm->next_ref_frame_map));
+  // cm->next_ref_frame_map, so we can directly reset it to all NULL.
+  for (i = 0; i < REF_FRAMES; ++i) {
+    cm->next_ref_frame_map[i] = NULL;
+  }
 
   lock_buffer_pool(cm->buffer_pool);
   reset_ref_frame_map(cm);

av1/decoder/decoder.c

diff --git a/av1/decoder/decoder.c b/av1/decoder/decoder.c
index 31edd1d..f8b598c 100644
--- a/av1/decoder/decoder.c
+++ b/av1/decoder/decoder.c
@@ -330,8 +330,8 @@
   if (pbi->hold_ref_buf) {
     for (int ref_index = 0; ref_index < REF_FRAMES; ++ref_index) {
       decrease_ref_count(cm->next_ref_frame_map[ref_index], pool);
+      cm->next_ref_frame_map[ref_index] = NULL;
     }
-    memset(&cm->next_ref_frame_map, -1, sizeof(cm->next_ref_frame_map));
     pbi->hold_ref_buf = 0;
   }
   // Release current frame.