nohz: Reset next_tick cache even when the timer has no regs

Handle tick interrupts whose regs are NULL, out of general paranoia. It happens
when hrtimer_interrupt() is called from non-interrupt contexts, such as hotplug
CPU down events.

Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>