event: Be more defensive on category and platform

Both category and platform can be missing, or only one of the two be
set. Default both to unknown, and only when we get two non-empty matches
after splitting on the '—' do we fill them in.

event.go

@@ -45,8 +45,18 @@ func eventFromHTML(rows ...soup.Root) (*Event, error) {
 
 	e.Estimate = toDuration(tr2[0].Text())
 	catPlat := strings.Split(tr2[1].Text(), "—") // Ceci n'est pas un -
-	e.Category = strings.TrimSpace(catPlat[0])
-	e.Platform = strings.TrimSpace(catPlat[1])
+	category := "unknown"
+	platform := "unknown"
+	if len(catPlat) == 2 {
+		if c := strings.TrimSpace(catPlat[0]); c != "" {
+			category = c
+		}
+		if p := strings.TrimSpace(catPlat[1]); p != "" {
+			platform = p
+		}
+	}
+	e.Category = category
+	e.Platform = platform
 	e.Hosts = nicksToSlice(tr2[2].Text())
 
 	return e, nil