added assertion to help suppress clang warnings

Author: patcheng

include/rapidjson/internal/stack.h

@@ -113,6 +113,7 @@ class Stack {
     // Expansion is run very infrequently, so it is moved to another (probably non-inline) function.
     template<typename T>
     RAPIDJSON_FORCEINLINE void Reserve(size_t count = 1) {
+        RAPIDJSON_ASSERT(stackTop_);
          // Expand the stack if needed
         if (RAPIDJSON_UNLIKELY(stackTop_ + sizeof(T) * count > stackEnd_))
             Expand<T>(count);
@@ -126,6 +127,7 @@ class Stack {
 
     template<typename T>
     RAPIDJSON_FORCEINLINE T* PushUnsafe(size_t count = 1) {
+        RAPIDJSON_ASSERT(stackTop_);
         RAPIDJSON_ASSERT(stackTop_ + sizeof(T) * count <= stackEnd_);
         T* ret = reinterpret_cast<T*>(stackTop_);
         stackTop_ += sizeof(T) * count;