During the first update of the hw_clear_state atoms, we may not yet
have a current rasterizer state object. So, svga->curr.rast may be
NULL and we crash.
Add a few null pointer checks to work around this. Note that these
are only needed in the state update functions which are called for
'clear' validation.
Reviewed-by: Charmaine Lee <charmainel@vmware.com>