Fix: reversed logic in packet vs content size

As described in CTF section 5.2

"If the packet size field is missing, the whole stream only contains a
single packet. If the content size field is missing, the packet is
filled (no padding). The content and packet sizes include all headers."

Here is the correct semantic:

* Content size and packet size are available:

  Packet is filled with data up to content size, and then with padding
  up to packet size.

* Content size available, no packet size field:

  The stream has a single packet. It is filled up to content size, and
  the rest is padding.

* Packet size available, no content size field:

  Packet filled completely, no padding.

Fixes #683

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>