Disambiguate pkt-line "0000" from "0004"
The pkt-line length includes its own 4 bytes. So "0000" is the
magic flush/end marker used as part of the protocols based upon
pkt-line, while "0004" indicates a packet of 0 bytes, but not a
flush/end marker.
Currently there is no need for this distinction in the code, as
the protocol never sends an empty packet, but it reduces the risk
that in the future a "0004" packet is misread as a "0000" flush.
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Robin Rosenberg <robin.rosenberg@dewire.com>