Fix for AlarmClock(): Prevent freeze-up when time <=0 and loop = true

Small fix for a potential Freeze-up:
ATM xbmc freezes when we call AlarmClock() with time <= 0 and loop = true.
for skins this could happen when we allow the user to set the AlarmClock loop time with Skin.SetNumeric and then call
 "AlarmClock(name,Notification(test,test),$INFO[Skin.String(Time)],loop=true)"
 with "Time" <= 0. 
if that call is done in startup/main menu window then the user has no chance to change the skin / fix the invalid number.
This commit fixes that and prevents the alarmclock call for those invalid time values.