Fix: Off by one in seq num for data pending command

Like the close stream command, the next sequence number of the stream
needs to be used minus 1 for the data pending or else we are off by one
on the relayd during the check since 4 data packets for instance means a
prev_seq value of 4 but a last_next_seq_num of 5 hence creating an off
by one for the data pending check.

Furthermore, the check was actually wrong on the relayd side. Having a
previous sequence number lower than the last one seen does NOT mean that
the data is not pending so the check needed was actually equal or
greater.

Signed-off-by: David Goulet <dgoulet@efficios.com>