Reduce the size of the seen_bufs array

Reduce the size of the seen_bufs array in
check_frame_refs_short_signaling(). Its size is at most the number of
reference types from LAST_FRAME to ALTREF_FRAME, inclusive. That number
is INTER_REFS_PER_FRAME.

Change-Id: Ie6a59564ad4f2c4c0c2b4de15ca6aa87ed547b70

av1/encoder/bitstream.c

diff --git a/av1/encoder/bitstream.c b/av1/encoder/bitstream.c
index 4f85307..39aa027 100644
--- a/av1/encoder/bitstream.c
+++ b/av1/encoder/bitstream.c
@@ -2795,7 +2795,7 @@
   }
 
   // Check whether all references are distinct frames.
-  const RefCntBuffer *seen_bufs[FRAME_BUFFERS] = { NULL };
+  const RefCntBuffer *seen_bufs[INTER_REFS_PER_FRAME] = { NULL };
   int num_refs = 0;
   for (int ref_frame = LAST_FRAME; ref_frame <= ALTREF_FRAME; ++ref_frame) {
     const RefCntBuffer *const buf = get_ref_frame_buf(cm, ref_frame);