AdHocCache: explicate chunk_incref assertion

src/borg/cache.py

@@ -1001,6 +1001,9 @@ Chunk index:    {0.total_unique_chunks:20d}             unknown"""
             self._begin_txn()
         count, size, csize = self.chunks.incref(id)
         stats.update(size or size_, csize, False)
+        # When size is 0 and size_ is not given, then this chunk has not been locally visited yet (seen_chunk with
+        # size or add_chunk); we can't add references to those (size=0 is invalid) and generally don't try to.
+        assert size or size_
         return ChunkListEntry(id, size or size_, csize)
 
     def chunk_decref(self, id, stats, wait=True):