Avoid calling memcpy with NULL pointers

According to the C/C++ standards, calling `memcpy(NULL, NULL, 0)` is
undefined behaviour. Recent GCC versions may rely on this by optimizing
NULL pointer checks more aggressively, see [1].

This patch tries to avoid calling std::memcpy with zero elements.
As a side effect, explicitly return NULL when requesting an empty block
from MemoryPoolAllocator::Malloc.

This may be related to #301.

[1] https://gcc.gnu.org/gcc-4.9/porting_to.html

include/rapidjson/allocators.h

@@ -160,6 +160,9 @@ public:
 
     //! Allocates a memory block. (concept Allocator)
     void* Malloc(size_t size) {
+        if (!size)
+            return NULL;
+
         size = RAPIDJSON_ALIGN(size);
         if (chunkHead_ == 0 || chunkHead_->size + size > chunkHead_->capacity)
             AddChunk(chunk_capacity_ > size ? chunk_capacity_ : size);
@@ -191,7 +194,9 @@ public:
         // Realloc process: allocate and copy memory, do not free original buffer.
         void* newBuffer = Malloc(newSize);
         RAPIDJSON_ASSERT(newBuffer != 0);   // Do not handle out-of-memory explicitly.
-        return std::memcpy(newBuffer, originalPtr, originalSize);
+        if (originalSize)
+            std::memcpy(newBuffer, originalPtr, originalSize);
+        return newBuffer;
     }
 
     //! Frees a memory block (concept Allocator)

include/rapidjson/document.h

@@ -1582,16 +1582,24 @@ private:
     // Initialize this value as array with initial data, without calling destructor.
     void SetArrayRaw(GenericValue* values, SizeType count, Allocator& allocator) {
         flags_ = kArrayFlag;
+        if (count) {
             data_.a.elements = (GenericValue*)allocator.Malloc(count * sizeof(GenericValue));
             std::memcpy(data_.a.elements, values, count * sizeof(GenericValue));
+        }
+        else
+            data_.a.elements = NULL;
         data_.a.size = data_.a.capacity = count;
     }
 
     //! Initialize this value as object with initial data, without calling destructor.
     void SetObjectRaw(Member* members, SizeType count, Allocator& allocator) {
         flags_ = kObjectFlag;
+        if (count) {
             data_.o.members = (Member*)allocator.Malloc(count * sizeof(Member));
             std::memcpy(data_.o.members, members, count * sizeof(Member));
+        }
+        else
+            data_.o.members = NULL;
         data_.o.size = data_.o.capacity = count;
     }