fixed dissector for packets with inner IPv4/IPv6 packets

[hakanurhan]

The dissector was having problems to dissect the packets when there are inner IPv4/IPv6 packets within the UDP payload (eg. SIP packets). This small fix makes them to be dissected properly.

[peremen]

Thanks for the effort, but I don't think this commit is needed. By default, SCAT sends IP packets to the UDP port 47290. In the very end of the file, UDP port 47290 uses ip_dissector instead of gsmtap_wrapper_proto. Therefore there should be no need to handle the IP packets within the GSMTAP dissector. If you've found the place where IP packet is sent to the UDP port 4729, then it is SCAT's bug which should be fixed in the main code.