Take advantage of implicit std::unique_ptr<>(nulltpr_t) ctor.

xfa/fgas/crt/fgas_memory.cpp

 // Copyright 2014 PDFium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 // Original code copyright 2014 Foxit Software Inc. http://www.foxitsoftware.com
 
 #include "xfa/fgas/crt/fgas_memory.h"
 
 #ifndef MEMORY_TOOL_REPLACES_ALLOCATOR
 // Use CFX_DefStore to replace CFX_FixedStore to simplify memory
@@ skipping 82 matching lines @@
           new CFX_StaticStore(chunkSize));
     case FX_ALLOCTYPE_Fixed:
 #ifdef MEMORY_TOOL_REPLACES_ALLOCATOR
       return std::unique_ptr<IFX_MemoryAllocator>(new CFX_DefStore());
 #else
       return std::unique_ptr<IFX_MemoryAllocator>(
           new CFX_FixedStore(blockSize, chunkSize));
 #endif  // MEMORY_TOOL_REPLACES_ALLOCATOR
     default:
       ASSERT(0);
-      return std::unique_ptr<IFX_MemoryAllocator>();
+      return nullptr;
   }
 }
 
 CFX_StaticStore::CFX_StaticStore(size_t iDefChunkSize)
     : m_iAllocatedSize(0),
       m_iDefChunkSize(iDefChunkSize),
       m_pChunk(nullptr),
       m_pLastChunk(nullptr) {
   ASSERT(m_iDefChunkSize != 0);
 }
@@ skipping 131 matching lines @@
     if (!pPrior) {
       m_pChunk = pChunk->pNextChunk;
     } else {
       pPrior->pNextChunk = pChunk->pNextChunk;
     }
     FX_Free(pChunk);
   }
 }
 
 #endif  // MEMORY_TOOL_REPLACES_ALLOCATOR