MemoryPoolAllocator, Stack: lazily allocate Allocators

In order to make the constructors more efficient, especially
in the context of C++11 move semantics, the (dynamic) allocations
in MemoryPoolAllocator and Stack should be performed lazily.

Move the allocations to the first use of the allocator in both
classes.

include/rapidjson/allocators.h

@@ -104,9 +104,6 @@ public:
     MemoryPoolAllocator(size_t chunkSize = kDefaultChunkCapacity, BaseAllocator* baseAllocator = 0) : 
         chunkHead_(0), chunk_capacity_(chunkSize), userBuffer_(0), baseAllocator_(baseAllocator), ownBaseAllocator_(0)
     {
-        if (!baseAllocator_)
-            ownBaseAllocator_ = baseAllocator_ = RAPIDJSON_NEW(BaseAllocator());
-        AddChunk(chunk_capacity_);
     }
 
     //! Constructor with user-supplied buffer.
@@ -216,6 +213,8 @@ private:
     /*! \param capacity Capacity of the chunk in bytes.
     */
     void AddChunk(size_t capacity) {
+        if (!baseAllocator_)
+            ownBaseAllocator_ = baseAllocator_ = RAPIDJSON_NEW(BaseAllocator());
         ChunkHeader* chunk = reinterpret_cast<ChunkHeader*>(baseAllocator_->Malloc(sizeof(ChunkHeader) + capacity));
         chunk->capacity = capacity;
         chunk->size = 0;

include/rapidjson/internal/stack.h

@@ -37,8 +37,6 @@ public:
     // Do it lazily when first Push() -> Expand() -> Resize().
     Stack(Allocator* allocator, size_t stackCapacity) : allocator_(allocator), ownAllocator_(0), stack_(0), stackTop_(0), stackEnd_(0), initialCapacity_(stackCapacity) {
         RAPIDJSON_ASSERT(stackCapacity > 0);
-        if (!allocator_)
-            ownAllocator_ = allocator_ = RAPIDJSON_NEW(Allocator());
     }
 
 #if RAPIDJSON_HAS_CXX11_RVALUE_REFS
@@ -140,9 +138,11 @@ private:
     void Expand(size_t count) {
         // Only expand the capacity if the current stack exists. Otherwise just create a stack with initial capacity.
         size_t newCapacity;
-        if (stack_ == 0)
+        if (stack_ == 0) {
+            if (!allocator_)
+                ownAllocator_ = allocator_ = RAPIDJSON_NEW(Allocator());
             newCapacity = initialCapacity_;
-        else {
+        } else {
             newCapacity = GetCapacity();
             newCapacity += (newCapacity + 1) / 2;
         }