rx: connection aborts send serial zero when no conn available

rx: connection aborts send serial zero when no conn available

When no connection object is available, send serial number zero (0)
instead of one (1).  There is no harm in sending one (1) but it might
be confused as the first packet sent on the connection.  Multiple
connection aborts sent would all be sent with serial one (1).

Serial number zero (0) can be an indication to humans reading packet
traces that the sender has no knowledge of the connection.

Change-Id: I1951284f810170bd130e4f1d8ed93b903cd66659
Reviewed-on: https://gerrit.openafs.org/12932
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>