Database upgrade 16: fix event edits.

Fixes #198.

etc/db/schema-template.sql

@@ -399,7 +399,15 @@ CREATE FUNCTION _SURVEY__TEMPLATE_.event_log_update() RETURNS trigger
 			-- Any attempt to modify id will be ignored.
 			INSERT INTO event_log_full
 				(id, tstamp, sequence, point, remarks, labels, meta)
-			VALUES (OLD.id, NEW.tstamp, NEW.sequence, NEW.point, NEW.remarks, NEW.labels, NEW.meta);
+			VALUES (
+				OLD.id,
+				COALESCE(NEW.tstamp, OLD.tstamp),
+				COALESCE(NEW.sequence, OLD.sequence),
+				COALESCE(NEW.point, OLD.point),
+				COALESCE(NEW.remarks, OLD.remarks),
+				COALESCE(NEW.labels, OLD.labels),
+				COALESCE(NEW.meta, OLD.meta)
+			);
 
 			RETURN NEW;