loader: free error state, when checking the drawable type

Currently we distinguish if the drawable is a window or pixmap by
checking xcb_present_select_input throws an error or not.

Yet, we don't always free the error state returned by xcb.

Cc: Kirill Burtsev <kirill.burtsev@qt.io>
Cc: Boyan Ding <boyan.j.ding@gmail.com>
Fixes: 6bd9ba7d ("loader: Add dri3 helper")
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
[Emil: add commit message, fixes tag]
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
(cherry picked from commit a5393164)