[bug] UDP::matches_response() return 0 when no inner PDU is available

[markvmk]

Hi,

When trying to match against UDP response that has no payload the UDP::matches_response() fails (returns false).
https://github.com/mfontanini/libtins/blob/master/src/udp.cpp#L173

For EthernetII and IP PDUs the implementation of matches_response() returns true in case no inner PDU is present.
I am not sure if the UDP implementation is intentionally done like that but as I could not find this in the documentation I assume that this is a bug and UDP::matches_response() should also return true when inner PDU is not present.