avcodec/cbs_av1: infer frame_type when parsing a show_existing_frame frame

Reviewed-by: Mark Thompson <sw@jkqxz.net>
Signed-off-by: James Almer <jamrial@gmail.com>
(cherry picked from commit 6c20207dce)

libavcodec/cbs_av1_syntax_template.c

@@ -1280,6 +1280,7 @@ static int FUNC(uncompressed_header)(CodedBitstreamContext *ctx, RWContext *rw,
             else
                 infer(refresh_frame_flags, 0);
 
+            infer(frame_type,            frame->frame_type);
             return 0;
         }