Show stderr output from tcpdump other than "reading from file...".

[tcpdump] / tests / cve2015-0261-ipv6.out.stderr

reading from file cve2015-0261-ipv6.pcap, link-type SLIP (SLIP), snapshot length 65535
tcpdump: pcap_loop: invalid packet capture length 268463617, bigger than snaplen of 65535