svga: Fix debug build crash when rendering to depth/stencil renderbuffer only.

src/gallium/drivers/svga/svga_draw.c

@@ -164,7 +164,8 @@ svga_hwtnl_flush( struct svga_hwtnl *hwtnl )
       }
 
       SVGA_DBG(DEBUG_DMA, "draw to sid %p, %d prims\n",
-               svga_surface(svga->curr.framebuffer.cbufs[0])->handle,
+               svga->curr.framebuffer.cbufs[0] ?
+               svga_surface(svga->curr.framebuffer.cbufs[0])->handle : NULL,
                hwtnl->cmd.prim_count);
 
       ret = SVGA3D_BeginDrawPrimitives(swc,