QNX: Query dynamic buffer count at runtime

QNX: Query dynamic buffer count at runtime

While unlikely, there are cases in which QQnxWindow::renderBuffer() is
called before the window buffers have been created. Without this check, the
program will abort on QQnxBuffer constructor, since the value that will be
passed to it will be of an invalid buffer.

Change-Id: I9ad5926dca856570032dcf10b6975e8f3364c284
Reviewed-by: Thomas McGuire <thomas.mcguire@kdab.com>
Reviewed-by: Sean Harmer <sean.harmer@kdab.com>