fix Traceback when running check --repair, attic issue #232

This fix is maybe not perfect yet, but maybe better than nothing.

A comment by Ernest0x (see https://github.com/jborg/attic/issues/232 ):

@ThomasWaldmann your patch did the job.
attic check --repair did the repairing and attic delete deleted the archive.
Thanks.

That said, however, I am not sure if the best place to put the check is where
you put it in the patch. For example, the check operation uses a custom msgpack
unpacker class named "RobustUnpacker", which it does try to check for correct
format (see the comment: "Abort early if the data does not look like a
serialized dict"), but it seems it does not catch my case. The relevant code
in 'cache.py', on the other hand, uses msgpack's Unpacker class.

borg/archive.py

@@ -755,6 +755,10 @@ class ArchiveChecker:
                 for chunk_id, cdata in zip(items, repository.get_many(items)):
                     unpacker.feed(self.key.decrypt(chunk_id, cdata))
                     for item in unpacker:
+                        if not isinstance(item, dict):
+                            self.report_progress('Did not get expected metadata dict - archive corrupted!',
+                                                 error=True)
+                            continue
                         yield item
 
         repository = cache_if_remote(self.repository)

borg/cache.py

@@ -287,6 +287,9 @@ class Cache:
                 add(chunk_idx, item_id, len(data), len(chunk))
                 unpacker.feed(data)
                 for item in unpacker:
+                    if not isinstance(item, dict):
+                        print('Error: Did not get expected metadata dict - archive corrupted!')
+                        continue
                     if b'chunks' in item:
                         for chunk_id, size, csize in item[b'chunks']:
                             add(chunk_idx, chunk_id, size, csize)