KVM: PIT: fix kpit_elapsed division by zero

Fix division by zero triggered by latch count command on uninitialized
counter.

Cc: [e-mail]
Signed-off-by: Marcelo Tosatti <[e-mail]>
Signed-off-by: Avi Kivity <[e-mail]>

arch/x86/kvm/i8254.c

···
 	ktime_t remaining;
 	struct kvm_kpit_state *ps = &kvm->arch.vpit->pit_state;
 
+	if (!ps->pit_timer.period)
+		return 0;
+
 	/*
 	 * The Counter does not stop when it reaches zero. In
 	 * Modes 0, 1, 4, and 5 the Counter ``wraps around'' to