Ensure range of month is correct for next emergency day

Because we count in months and not years in world.lua any index over 12 will cause nil returning 
from the month_length table when deciding the next day the emergency is scheduled for. 
This change calculates the actual month in the range 1-12 from the current emergency month count. 

This bug managed to survive because there was up to this point only a timer when the hospital was opening
so the months was in the range 1-12, but for something else which uses a timer we would get a nil exception
when trying to use the result.