reset file position to start after reading block index

Author: braingram

asdf/_block/reader.py

@@ -247,4 +247,5 @@ def read_blocks(fd, memmap=False, lazy_load=False, validate_checksums=False, aft
         return _read_blocks_serially(fd, memmap, lazy_load, after_magic)
 
     # skip magic for each block
+    fd.seek(starting_offset)
     return [ReadBlock(offset + magic_len, fd, memmap, lazy_load, validate_checksums) for offset in block_index]