rtc: check if __rtc_read_time was successful in rtc_timer_do_work()

In the Linux kernel, the following vulnerability has been resolved:

rtc: check if __rtc_read_time was successful in rtc_timer_do_work()

If the __rtc_read_time call fails,, the struct rtc_time tm; may contain

uninitialized data, or an illegal date/time read from the RTC hardware.

When calling rtc_tm_to_ktime later, the result may be a very large value

(possibly KTIME_MAX). If there are periodic timers in rtc->timerqueue,

they will continually expire, may causing kernel softlockup.