Merge pull request Tencent#727 from mapbox/silence-dereference-null-pointer

Silence false positive clang-tidy warning

Author: miloyip

include/rapidjson/internal/stack.h

@@ -17,6 +17,7 @@
 
 #include "../allocators.h"
 #include "swap.h"
+#include <cstddef>
 
 #if defined(__clang__)
 RAPIDJSON_DIAG_PUSH
@@ -114,7 +115,7 @@ class Stack {
     template<typename T>
     RAPIDJSON_FORCEINLINE void Reserve(size_t count = 1) {
          // Expand the stack if needed
-        if (RAPIDJSON_UNLIKELY(stackTop_ + sizeof(T) * count > stackEnd_))
+        if (RAPIDJSON_UNLIKELY(static_cast<std::ptrdiff_t>(sizeof(T) * count) > (stackEnd_ - stackTop_)))
             Expand<T>(count);
     }
 
@@ -127,7 +128,7 @@ class Stack {
     template<typename T>
     RAPIDJSON_FORCEINLINE T* PushUnsafe(size_t count = 1) {
         RAPIDJSON_ASSERT(stackTop_);
-        RAPIDJSON_ASSERT(stackTop_ + sizeof(T) * count <= stackEnd_);
+        RAPIDJSON_ASSERT(static_cast<std::ptrdiff_t>(sizeof(T) * count) <= (stackEnd_ - stackTop_));
         T* ret = reinterpret_cast<T*>(stackTop_);
         stackTop_ += sizeof(T) * count;
         return ret;