HDFS-15232. Fix libhdfspp test failures with GCC 7. (#1906)

(cherry picked from commit f59f6891c8bf70bdddc1f2535588bea41155b480)

hadoop-hdfs-project/hadoop-hdfs-native-client/src/main/native/libhdfspp/third_party/gmock-1.7.0/gmock/gmock.h

@@ -9930,6 +9930,8 @@ class ActionResultHolder : public UntypedActionResultHolderBase {
 template <>
 class ActionResultHolder<void> : public UntypedActionResultHolderBase {
  public:
+  explicit ActionResultHolder() {}
+
   void GetValueAndDelete() const { delete this; }
 
   virtual void PrintAsActionResult(::std::ostream* /* os */) const {}
@@ -9941,7 +9943,7 @@ class ActionResultHolder<void> : public UntypedActionResultHolderBase {
       const typename Function<F>::ArgumentTuple& args,
       const string& call_description) {
     func_mocker->PerformDefaultAction(args, call_description);
-    return NULL;
+    return new ActionResultHolder();
   }
 
   // Performs the given action and returns NULL.
@@ -9950,7 +9952,7 @@ class ActionResultHolder<void> : public UntypedActionResultHolderBase {
       const Action<F>& action,
       const typename Function<F>::ArgumentTuple& args) {
     action.Perform(args);
-    return NULL;
+    return new ActionResultHolder();
   }
 };