Check packet has data via buffer size not PSH

The PSH flag tells the receiver to flush it's buffer. While this will
never be set for packets that have no data it can be, and often is, not
set for packets that do. For example, when a message spans more than one
segment the earlier segments often do not have the PSH flag set.

This alternative solution checks that the buffer is larger than the TCP
header.
Merge pull request #63 from robyoung/check-packet-size

Check packet has data via buffer size not PSH