util/StringAPI: add memrchr() wrapper

src/util/WStringAPI.hxx

@@ -90,6 +90,21 @@ StringFindLast(wchar_t *haystack, wchar_t needle) noexcept
 	return wcsrchr(haystack, needle);
 }
 
+gcc_pure gcc_nonnull_all
+static inline const wchar_t *
+StringFindLast(const wchar_t *haystack, wchar_t needle, size_t size) noexcept
+{
+	/* there's no wmemrchr() unfortunately */
+	const auto *p = haystack + size;
+	while (p > haystack) {
+		--p;
+		if (*p == needle)
+			return p;
+	}
+
+	return nullptr;
+}
+
 gcc_pure gcc_nonnull_all
 static inline const wchar_t *
 StringFindAny(const wchar_t *haystack, const wchar_t *accept) noexcept