Using FMT_NULLPTR instead of literal 0

(cherry picked from commit 49ccb2e449)

test/mock-allocator.h

@@ -36,7 +36,7 @@ class MockAllocator {
   MockAllocator() {}
   MockAllocator(const MockAllocator &) {}
   typedef T value_type;
-  MOCK_METHOD1_T(allocate, T* (std::size_t n));
+  MOCK_METHOD2_T(allocate, T* (std::size_t n, const T* h));
   MOCK_METHOD2_T(deallocate, void (T* p, std::size_t n));
 };
 
@@ -78,7 +78,7 @@ class AllocatorRef {
 
   Allocator *get() const { return alloc_; }
 
-  value_type* allocate(std::size_t n) { return alloc_->allocate(n); }
+  value_type* allocate(std::size_t n,  const value_type* h) { return alloc_->allocate(n, h); }
   void deallocate(value_type* p, std::size_t n) { alloc_->deallocate(p, n); }
 };