[apps/FibChildStealing] Improve fibNum arg parsing

Also, don't set an unsigned to -1…

apps/FibChildStealing.cpp

@@ -66,13 +66,13 @@ static void fib(void* voidParams) {
 
 // NOLINTNEXTLINE(bugprone-exception-escape)
 auto main(int argc, char* argv[]) -> int {
-	uint64_t fibNum = -1;
+	uint64_t fibNum = 12;
 	po::options_description desc("Allowed options");
 	// clang-format off
 	desc.add_options()
 		("help", "Show help")
 		("nthreads", po::value<unsigned int>()->default_value(std::thread::hardware_concurrency()), "Number of worker threads used by EMPER's runtime system")
-		("fibnum", po::value<uint64_t>(&fibNum)->default_value(12), "The Fibonacci number to compute")
+		("fibnum", po::value<uint64_t>(&fibNum)->default_value(fibNum), "The Fibonacci number to compute")
 		;
 	// clang-format on