util: use UINT64_MAX

Since we are checking the size of long and long long, use UINT64_MAX in
the fallback to be sure thre right size is being used.
diff --git a/shared/util.h b/shared/util.h
index 6f602d3..c6a31df 100644
--- a/shared/util.h
+++ b/shared/util.h
@@ -86,5 +86,5 @@
 #endif
 #endif
 	*res = a + b;
-	return ULLONG_MAX - a < b;
+	return UINT64_MAX - a < b;
 }