WPS: Check for theoretical gmtime() failure

In theory, gmtime() could return NULL if the year value would not fit
into an integer. However, that cannot really happen with the current
time() value in practice. Anyway, clean up static analyzer reports by
checking for this corner case.

Signed-off-by: Jouni Malinen <j@w1.fi>

src/wps/wps_upnp.c

@@ -227,6 +227,8 @@ void format_date(struct wpabuf *buf)
 
 	t = time(NULL);
 	date = gmtime(&t);
+	if (date == NULL)
+		return;
 	wpabuf_printf(buf, "%s, %02d %s %d %02d:%02d:%02d GMT",
 		      &weekday_str[date->tm_wday * 4], date->tm_mday,
 		      &month_str[date->tm_mon * 4], date->tm_year + 1900,