flowtop: set bug if screen is NULL

If this is ever the case, bail out.

Signed-off-by: Daniel Borkmann <dborkman@redhat.com>
netsniff-ng: walk_t3_block: prevent fd from double close

Only close the very first pcap file of multi-pcap files once, and not
once during next_multi_pcap_file and once during exit.

Discovered by Coverty scanner.

Signed-off-by: Daniel Borkmann <dborkman@redhat.com>