src/client/http_client.rs: use start of chunk as offset

src/api2/admin/datastore/backup/environment.rs

@@ -158,14 +158,15 @@ impl BackupEnvironment {
             None => bail!("dynamic writer '{}' not registered", wid),
         };
 
-        data.offset += size as u64;
-        data.chunk_count += 1;
 
         if data.offset != offset {
             bail!("dynamic writer '{}' append chunk failed - got strange chunk offset ({} != {})",
                   data.name, data.offset, offset);
         }
 
+        data.offset += size as u64;
+        data.chunk_count += 1;
+
         data.index.add_chunk(data.offset, digest)?;
 
         Ok(())