util/{Const,Writable}Buffer: add operator[]

src/util/ConstBuffer.hxx

@@ -143,6 +143,17 @@ struct ConstBuffer {
 	constexpr const_iterator cend() const {
 		return data + size;
 	}
+
+#ifdef NDEBUG
+	constexpr
+#endif
+	const T &operator[](size_type i) const {
+#ifndef NDEBUG
+		assert(i < size);
+#endif
+
+		return data[i];
+	}
 };
 
 #endif