Check for return value of ConsumeWithBuffer

Test: treehugger
Bug: 168554689

Change-Id: I1447c705143fe3d88267dcd08bbc9674d9cc6676
diff --git a/payload_consumer/snapshot_extent_writer.h b/payload_consumer/snapshot_extent_writer.h
index fb4b548..6d9fe7d 100644
--- a/payload_consumer/snapshot_extent_writer.h
+++ b/payload_consumer/snapshot_extent_writer.h
@@ -41,7 +41,7 @@
 
  private:
   bool next_extent();
-  size_t ConsumeWithBuffer(const uint8_t* bytes, size_t count);
+  [[nodiscard]] size_t ConsumeWithBuffer(const uint8_t* bytes, size_t count);
   // It's a non-owning pointer, because PartitionWriter owns the CowWruter. This
   // allows us to use a single instance of CowWriter for all operations applied
   // to the same partition.