Suppress invalid or non-meaningful statistics - for example, without

Suppress invalid or non-meaningful statistics - for example, without
time stamps on all packets in a set, you can't determine the start and
end time of the packets in the set (even one timestampless packet throws
the determination off - was that packet before the first time-stamped or
after the last time-stamped packet, or between them?).

svn path=/trunk/; revision=41187