check for default protocol rx buf limit

This fixes

http://libwebsockets.org/trac/ticket/13

When using the default rx protocol buffer, the check is
performed against 0 not the default length.  That's the
case both in client and server code...

There's no problem if you actually give a max frame size
in the protocol definition.

Signed-off-by: Andy Green <andy.green@linaro.org>