Mark failed-to-decode frames as incomplete when --maxframedelay=1

Credit to oss-fuzz.

src/decode.c

@@ -3425,6 +3425,7 @@ void dav1d_decode_frame_exit(Dav1dFrameContext *const f, const int retval) {
 
     for (int i = 0; i < f->n_tile_data; i++)
         dav1d_data_unref_internal(&f->tile[i].data);
+    f->task_thread.retval = retval;
 }
 
 int dav1d_decode_frame(Dav1dFrameContext *const f) {
@@ -3448,6 +3449,7 @@ int dav1d_decode_frame(Dav1dFrameContext *const f) {
                 }
             }
             pthread_mutex_unlock(&f->task_thread.ttd->lock);
+            res = f->task_thread.retval;
         } else {
             res = dav1d_decode_frame_main(f);
             if (!res && f->frame_hdr->refresh_context && f->task_thread.update_set) {

src/internal.h

@@ -302,6 +302,7 @@ struct Dav1dFrameContext {
         struct Dav1dTask *tasks, *tile_tasks[2], init_task;
         int num_tasks, num_tile_tasks;
         int done[2];
+        int retval;
         int update_set; // whether we need to update CDF reference
         atomic_int error;
         int task_counter;