Call SetOperationBlob in ChunkProcessor

BUG=None
TEST=FEATURES=test emerge-peppy update_engine

Change-Id: I0b946ce2ad945088f9f57cff4a13a1913e022769
diff --git a/payload_generator/annotated_operation.cc b/payload_generator/annotated_operation.cc
index 70350ed..3f28c8d 100644
--- a/payload_generator/annotated_operation.cc
+++ b/payload_generator/annotated_operation.cc
@@ -39,11 +39,10 @@
 
 bool AnnotatedOperation::SetOperationBlob(chromeos::Blob* blob,
                                           BlobFileWriter* blob_file) {
-  op.set_data_length(blob->size());
   off_t data_offset = blob_file->StoreBlob(*blob);
-  if (data_offset == -1)
-    return false;
+  TEST_AND_RETURN_FALSE(data_offset != -1);
   op.set_data_offset(data_offset);
+  op.set_data_length(blob->size());
   return true;
 }