ext_external_objects/vk-pix-buf-update-errors: data initialization fix

Used calloc instead of malloc to fill the data in piglit_display of vk_pix_buf_update_errors.c. These data will only be used to see if glBufferSubData will return an error and are of no importance.

Also moved the filling of the texture after the attempt to overwrite the buffer.

Signed-off-by: Eleni Maria Stea estea@igalia.com