Make sure ExtentRanges don't merge extents

When querying for XOR merge ops, we want the unmerged extents. E.g.
[5-9] and [10-14] should not be merged into 1 extent, because they could
correspond to different merge ops.

Test: th
Change-Id: I14c4c9fd6bf6361af6cd1e31264bdf860127a75b
diff --git a/payload_consumer/extent_map.h b/payload_consumer/extent_map.h
index 953eed0..b275e2d 100644
--- a/payload_consumer/extent_map.h
+++ b/payload_consumer/extent_map.h
@@ -89,7 +89,7 @@
  private:
   // Get a range of exents that potentially intersect with parameter |extent|
   std::map<Extent, T, Comparator> map_;
-  ExtentRanges set_;
+  ExtentRanges set_{false};
 };
 }  // namespace chromeos_update_engine