net-test: packetdrill encap support: free unknown sniffed packets

When we sniff an unknown packet, free it and allocate another packet
at the top of the sniffing loop. Previously we were not clearing state
after parsing an unknown packet, so state from the unkbown packet
could linger on, co-mingling with state from the next sniffed packet.


Change-Id: Ia9a330766969b114fd6084a087ff3f0d6b11d9d4