agent: Use 1280 instead of 65536 buffer size to send STUN Message

agent: Use 1280 instead of 65536 buffer size to send STUN Message

RFC 5389 says:
   All STUN messages sent over UDP SHOULD be less than the path MTU, if
   known.  If the path MTU is unknown, messages SHOULD be the smaller of
   576 bytes and the first-hop MTU for IPv4 [RFC1122] and 1280 bytes for
   IPv6 [RFC2460].

So sending 65536 bytes is always wrong