Avoid pointer arithmetic on null pointer to remove undefined behavior

The existing checks triggered undefined behavior when the stack was empty (null pointer). This change avoid this:
* If `stackTop_` and `stackEnd_` are null, it results in a `ptrdiff_t` of `0`
* If `stackTop_` and `stackEnd_` are valid pointers, they produce a `ptrdiff_t` with the remaining size on the stack

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 @@ public:
     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 @@ public:
     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;