Fix regression introduced in MDEV-19893

Some errors must be sent with seqno = 0, e.g those that are detected
before server sends its first "welcome" packet (e.g too many connections)
This was not taken into account originally in MDEV-19893 fix.

We need to check sql_errno, before fixing sequence number, to see
if the error we send is really an out-of-bound, e.g a KILL.