selftests/bpf: Don't close(-1) in serial_test_fexit_stress()

Guard close() with extra link_fd[i] > 0 and fexit_fd[i] > 0
check to prevent close(-1).

Signed-off-by: Ma Ke <make24@iscas.ac.cn>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20240623131753.2133829-1-make24@iscas.ac.cn

tools/testing/selftests/bpf/prog_tests/fexit_stress.c

@@ -50,9 +50,9 @@ void serial_test_fexit_stress(void)
 
 out:
 	for (i = 0; i < bpf_max_tramp_links; i++) {
-		if (link_fd[i])
+		if (link_fd[i] > 0)
 			close(link_fd[i]);
-		if (fexit_fd[i])
+		if (fexit_fd[i] > 0)
 			close(fexit_fd[i]);
 	}
 	free(fd);