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>