QNX: Move buffer count check to appropriate location

QNX: Move buffer count check to appropriate location

The buffer count check should take place right after buffers are created. For
some reason, the buffer count value inside libscreen may become incosistent during
the course of the program.

Change-Id: Icbbaf4734eac5b0c5c95bdd93771899f9fcdb7db
Reviewed-by: Kevin Krammer <kevin.krammer@kdab.com>
Reviewed-by: Thomas McGuire <thomas.mcguire@kdab.com>
Reviewed-by: Nicolas Arnaud-Cormos <nicolas@kdab.com>