(for 4.9.3) CVE-2018-14879/fix -V to fail invalid input safely

get_next_file() did not check the return value of strlen() and
underflowed an array index if the line read by fgets() from the file
started with \0. This caused an out-of-bounds read and could cause a
write. Add the missing check.

This vulnerability was discovered by Brian Carpenter & Geeknik Labs.
print_packet(u_char *user, const struct pcap_pkthdr *h, const u_char *sp)
{
	++packets_captured;

	++infodelay;

	pretty_print_packet((netdissect_options *)user, h, sp, packets_captured);

	--infodelay;
	if (infoprint)
		info(0);
}

print_packet(u_char *user, const struct pcap_pkthdr *h, const u_char *sp)
{
	++packets_captured;

	++infodelay;

	pretty_print_packet((netdissect_options *)user, h, sp, packets_captured);

	--infodelay;
	if (infoprint)
		info(0);
}
