Preempt possible errors when looking at historical data

src/analysis.rs

@@ -64,7 +64,14 @@ pub fn blocked_in(
     let mut blocked_in = HashSet::<String>::new();
     let today = date;
     for (country, info) in &bridge_info.info_by_country {
+        // If we haven't seen this bridge yet, return empty set
+        if today < info.first_seen {
+            return HashSet::<String>::new();
+        }
+
+        // (The part above prevents potential errors here.)
         let age = today - info.first_seen;
+
         if info.blocked {
             // Assume bridges never become unblocked
             blocked_in.insert(country.to_string());