Blob verify (#4951)

* Ensure signable data is not out of range

* Add a broadcast stage that puts bad sizes in blobs

* Resign blob after modifyign size

* Remove assertions that fail when size != meta.size

core/src/blocktree.rs

@@ -1490,8 +1490,6 @@ fn recover(
         }
     }
 
-    assert_ne!(size, 0);
-
     for i in start_idx..data_end_idx {
         let set_relative_idx = erasure_meta.data_index_in_set(i).unwrap() as usize;