Check the sample, flow, and counter lengths.

Check the sample, flow, and counter lengths.

When dissecting a sample, or a flow or counter within the sample, make
sure we don't run past the length in the sample/flow/counter header.

This also means we count the sample length down as we process the
contents of the sample, so we only add what, if anything, remains of the
sample to the pointer into the packet, and subtract that from the
remaining data length in the packet, which means we properly handle
packets with multiple samples.