Remove an unnecessary overflow check.

Remove an unnecessary overflow check.

We'll be running on a platform where int is 31 bits plus a sign bit
and size_t is either 32 bits or 64 bits, and the extra headers we put in
a "packet packet" will be much less than 2^31 bytes in size, so adding
the maximum packet size (an int, as pcap_snapshot() returns an int) to
the size of those extra headers won't overflow a size_t.