Don't crash when hash not in map.

cmd/mailhash/mailhash.go

@@ -123,7 +123,11 @@ func (m Messages) Reconcile(maildir string) error {
 
 		chksum := fmt.Sprintf("%x", h.Sum(nil))
 		md := email.NewInfo(path)
-		s := hashMap[chksum]
+		s, ok := hashMap[chksum]
+		if !ok {
+			return nil
+		}
+
 		glog.V(2).Infof("Comparing flags of %q to %q", path, s.Path)
 		if md.Seen != s.Read {
 			md.Seen = s.Read