pixman-renderer: Check if the shm_buffer is gone during attach

Moving attach to the render loop in c08a6ff8
caused this to crash if the shm buffer is destroyed between being committed
to a surface and being rendered.

Since we don't keep a reference to the buffer pool anyway, we might as well
just skip the attachment here.

Fixes: #922

Signed-off-by: Derek Foreman <derek.foreman@collabora.com>