Don't assume all packets have time stamps.

Pcap-ng files might have Simple Packet Blocks, which don't have time
stamps, and some other file formats might not have time stamps as well.

Only check the time stamp if we have one, and don't use "the start time
stamp is unset" as an indication that we've just read the first packet,
use "the count of packets we've read is 1" as an indication that we've
just read the first packet.

Change-Id: I097c54867655efb5c343fb880a113b3583736bbd
Reviewed-on: https://code.wireshark.org/review/1757
Reviewed-by: Guy Harris <guy@alum.mit.edu>

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