Add asserts to prevent UB

include/rapidjson/document.h

@@ -1989,7 +1989,10 @@ public:
     //!@}
 
     //! Get the allocator of this document.
-    Allocator& GetAllocator() { return *allocator_; }
+    Allocator& GetAllocator() {
+        RAPIDJSON_ASSERT(allocator_);
+        return *allocator_;
+    }
 
     //! Get the capacity of stack in bytes.
     size_t GetStackCapacity() const { return stack_.GetCapacity(); }

include/rapidjson/internal/stack.h

@@ -134,7 +134,10 @@ public:
     template<typename T>
     T* Bottom() { return (T*)stack_; }
 
-    Allocator& GetAllocator() { return *allocator_; }
+    Allocator& GetAllocator() {
+        RAPIDJSON_ASSERT(allocator_);
+        return *allocator_;
+    }
     bool Empty() const { return stackTop_ == stack_; }
     size_t GetSize() const { return static_cast<size_t>(stackTop_ - stack_); }
     size_t GetCapacity() const { return static_cast<size_t>(stackEnd_ - stack_); }