st/dri: fix crash when dri2_drawable_get_buffers fails

st/dri: fix crash when dri2_drawable_get_buffers fails

It's easily reproducible with Compiz with its Resize window mode
set to Normal (which is usually not the default mode).

https://bugs.freedesktop.org/show_bug.cgi?id=28658
https://bugs.freedesktop.org/show_bug.cgi?id=29303

This is actually a workaround to prevent Compiz crashes.
Instead, a completely white titlebar might show up during resizing
transparent windows (a rare case).

The underlying cause should be fixed by someone who has more knowledge
about the code. (dri2_drawable_get_buffers should not return NULL)

Acked-By: Jakob Bornecrantz <jakob@vmware.com>