fixup: tell() isn't helpful for OutOfData case

src/borg/fuse.py

@@ -149,14 +149,15 @@ def iter_archive_items(self, archive_item_ids, filter=None, consider_part_files=
             while True:
                 try:
                     item = unpacker.unpack()
+                    need_more_data = False
                 except msgpack.OutOfData:
                     need_more_data = True
-                else:
-                    need_more_data = False
 
                 start = stream_offset - chunk_begin
-                length = unpacker.tell() - stream_offset
-                stream_offset = stream_offset + length  # == tell()
+                # tell() is not helpful for the need_more_data case, but we know it is the remainder
+                # of the data in that case. in the other case, tell() works as expected.
+                length = (len(data) - start) if need_more_data else (unpacker.tell() - stream_offset)
+                stream_offset += length
                 msgpacked_bytes += data[start:start+length]
 
                 if need_more_data: