cbs_av1: fix incorrect data type

Since order_hint_bits_minus_1 range is 0~7, cur_frame_hint can be
most 128. And similar return value for cbs_av1_get_relative_dist.
So if plus them and use int8_t for the result may lose its precision.
Signed-off-by: Fei Wang <fei.w.wang@intel.com>

libavcodec/cbs_av1_syntax_template.c

@@ -355,7 +355,7 @@ static int FUNC(set_frame_refs)(CodedBitstreamContext *ctx, RWContext *rw,
         AV1_REF_FRAME_ALTREF2, AV1_REF_FRAME_ALTREF
     };
     int8_t ref_frame_idx[AV1_REFS_PER_FRAME], used_frame[AV1_NUM_REF_FRAMES];
-    int8_t shifted_order_hints[AV1_NUM_REF_FRAMES];
+    int16_t shifted_order_hints[AV1_NUM_REF_FRAMES];
     int cur_frame_hint, latest_order_hint, earliest_order_hint, ref;
     int i, j;