selftests/bpf: Sanitize pointer prior fclose()

There are scenarios where env.{sub,}test_state->stdout_saved, can be
NULL, e.g. sometimes when the watchdog timeout kicks in, or if the
open_memstream syscall is not available.

Avoid crashing test_progs by adding an explicit NULL check prior the
fclose() call.

Signed-off-by: Björn Töpel <bjorn@rivosinc.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Link: https://lore.kernel.org/bpf/20250318081648.122523-1-bjorn@kernel.org

Author: bjorn-rivos

tools/testing/selftests/bpf/test_progs.c

@@ -103,12 +103,14 @@ static void stdio_restore(void)
 	pthread_mutex_lock(&stdout_lock);
 
 	if (env.subtest_state) {
-		fclose(env.subtest_state->stdout_saved);
+		if (env.subtest_state->stdout_saved)
+			fclose(env.subtest_state->stdout_saved);
 		env.subtest_state->stdout_saved = NULL;
 		stdout = env.test_state->stdout_saved;
 		stderr = env.test_state->stdout_saved;
 	} else {
-		fclose(env.test_state->stdout_saved);
+		if (env.test_state->stdout_saved)
+			fclose(env.test_state->stdout_saved);
 		env.test_state->stdout_saved = NULL;
 		stdout = env.stdout_saved;
 		stderr = env.stderr_saved;