Standardize MemoryPoolAllocator::Realloc() also, and improve coverage

include/rapidjson/allocators.h

@@ -189,6 +189,9 @@ public:
         if (originalPtr == 0)
             return Malloc(newSize);
 
+        if (newSize == 0)
+            return NULL;
+
         // Do not shrink if new size is smaller than original
         if (originalSize >= newSize)
             return originalPtr;

test/unittest/allocatorstest.cpp

@@ -42,6 +42,9 @@ void TestAllocator(Allocator& a) {
         EXPECT_EQ(i, r[i]);
 
     Allocator::Free(r);
+
+    // Realloc to zero size
+    EXPECT_TRUE(a.Realloc(a.Malloc(1), 1, 0) == 0);
 }
 
 TEST(Allocator, CrtAllocator) {