net: tcp: Fix handling of keep-alive probes

Keep-alive probes have seq number set to SND.NXT - 1 and may or may not
contain an octet of data. The latter case was not handled properly
therefore add a special case when validating seq number to response to
keep-alive probes.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>
(cherry picked from commit d93c5d60)