nsexec: Check for errors in write_log()

First, check if strdup() fails and error out.

While we are there, the else case was missing brackets, as we only need
to check ret in the else case. Fix that too

Signed-off-by: Rodrigo Campos <rodrigoca@microsoft.com>

libcontainer/nsenter/nsexec.c

@@ -168,15 +168,17 @@ static void write_log(int level, const char *format, ...)
 
 	message = escape_json_string(message);
 
-	if (current_stage == STAGE_SETUP)
+	if (current_stage == STAGE_SETUP) {
 		stage = strdup("nsexec");
-	else
+		if (stage == NULL)
+			goto out;
+	} else {
 		ret = asprintf(&stage, "nsexec-%d", current_stage);
-	if (ret < 0) {
-		stage = NULL;
-		goto out;
+		if (ret < 0) {
+			stage = NULL;
+			goto out;
+		}
 	}
-
 	ret = asprintf(&json, "{\"level\":\"%s\", \"msg\": \"%s[%d]: %s\"}\n",
 		       level_str[level], stage, getpid(), message);
 	if (ret < 0) {