lib.c: clear cf after seeking

Fixes #390.
diff --git a/src/lib.c b/src/lib.c
index 032fafb..b4b4c97 100644
--- a/src/lib.c
+++ b/src/lib.c
@@ -580,12 +580,13 @@
         for (unsigned n = 0, next = c->frame_thread.next; n < c->n_fc; n++, next++) {
             if (next == c->n_fc) next = 0;
             Dav1dFrameContext *const f = &c->fc[next];
-            dav1d_decode_frame_exit(f, 0);
+            dav1d_decode_frame_exit(f, -1);
             f->n_tile_data = 0;
             Dav1dThreadPicture *out_delayed = &c->frame_thread.out_delayed[next];
             if (out_delayed->p.data[0]) {
                 dav1d_thread_picture_unref(out_delayed);
             }
+            f->task_thread.retval = 0;
         }
         c->frame_thread.next = 0;
     }