libweston-desktop/xdg_shell_v6: Check for buffer directly in committed
libweston-desktop/xdg_shell_v6: Check for buffer directly in committed

This is what we are interested in for real, and new_buffer is wrongly
named.

Signed-off-by: Quentin Glidic <241385a0e41ea713327e189bce1062decf7c1eb5@sardemff7.net>
Reviewed-by: Jonas Ådahl <67dcd8a17e758192e0cbb577ce1d62cc7da6cf49@gmail.com>

krezovic/weston,sir-murray/weston,krezovic/weston,sir-murray/weston