Mark remainder of rightmost page no_access after gc

VersionedMap.cpp

@@ -378,6 +378,9 @@ struct MemManager {
 #endif
     }
     next = max + 1;
+    assert(firstUnaddressable >= next);
+    VALGRIND_MAKE_MEM_NOACCESS(base + next,
+                               (firstUnaddressable - next) * sizeof(Node));
   }
 
   int64_t getBytes() const {