Move AdditionOverflow to util, Add CheckedAdd with unit tests

src/util/overflow.h

@@ -0,0 +1,31 @@
+// Copyright (c) 2021 The Bitcoin Core developers
+// Distributed under the MIT software license, see the accompanying
+// file COPYING or http://www.opensource.org/licenses/mit-license.php.
+
+#ifndef BITCOIN_UTIL_OVERFLOW_H
+#define BITCOIN_UTIL_OVERFLOW_H
+
+#include <limits>
+#include <type_traits>
+
+template <class T>
+[[nodiscard]] bool AdditionOverflow(const T i, const T j) noexcept
+{
+    static_assert(std::is_integral<T>::value, "Integral required.");
+    if (std::numeric_limits<T>::is_signed) {
+        return (i > 0 && j > std::numeric_limits<T>::max() - i) ||
+               (i < 0 && j < std::numeric_limits<T>::min() - i);
+    }
+    return std::numeric_limits<T>::max() - i < j;
+}
+
+template <class T>
+[[nodiscard]] std::optional<T> CheckedAdd(const T i, const T j) noexcept
+{
+    if (AdditionOverflow(i, j)) {
+        return std::nullopt;
+    }
+    return i + j;
+}
+
+#endif // BITCOIN_UTIL_OVERFLOW_H