fixed error handling for alarm description parsing

ical/Alarm.cpp

@@ -18,8 +18,10 @@ namespace ical {
   }
 
   void Alarm::parse_description(icalcomponent* alarm_component) {
-    // TODO Error Handling
-    description = icalproperty_get_value_as_string(icalcomponent_get_first_property(alarm_component, ICAL_DESCRIPTION_PROPERTY));
+    icalproperty* description_property = icalcomponent_get_first_property(alarm_component, ICAL_DESCRIPTION_PROPERTY);
+    if(description_property != 0) {
+      description = icalproperty_get_value_as_string(description_property);
+    }
   }
 
   void Alarm::parse_trigger(icalcomponent* alarm_component) {