fix handling non-integer arguments to +l

lib/irc/mode.ml

@@ -227,7 +227,7 @@ module Parse = struct
         begin
           try match m, arg with
             | `l, `set n ->
-              let n = int_of_string n in
+              let n = Option.get (int_of_string_opt n) in
               if n <= 0 then invalid_arg "limit <= 0";
               { acc with chan_limit = Some (`set n) }, args
             | `l, `unset ->