Check "latestRoot" property

VersionedMap.cpp

@@ -962,6 +962,10 @@ struct __attribute__((__visibility__("hidden"))) VersionedMap::Impl {
     }
     roots.add(latestRoot, latestVersion);
     totalMallocBytes += mallocBytesDelta;
+
+    // Check the "latestRoot is only meaningful in the callstack of
+    // addMutations" property
+    VALGRIND_MAKE_MEM_UNDEFINED(&latestRoot, sizeof(latestRoot));
   }
 
   struct StepwiseFirstGeq {