rx: rxi_FindRpcStat must test for empty queue

rx: rxi_FindRpcStat must test for empty queue

When queue_Scan is executed on an empty queue the queue element
variable, in this case 'rpc_stat' is the queue head, _RXQ(q),
and not NULL.  Callers of rxi_FindRpcStat() expect NULL on failure
to find or create an rx_interface_stat object.  Correct the behavior
by testing for an empty queue and return NULL immediately if the
queue is empty and the caller is not requesting creation.

Change-Id: I9952d7aaf357e039e87f6b8b17cb1a00208d6465
Reviewed-on: http://gerrit.openafs.org/7835
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>