r300g: Abort if draw_create() fails

The other drivers need to be updated to do this, too.

src/gallium/drivers/r300/r300_context.c

@@ -441,6 +441,8 @@ struct pipe_context* r300_create_context(struct pipe_screen* screen,
     if (!r300screen->caps.has_tcl) {
         /* Create a Draw. This is used for SW TCL. */
         r300->draw = draw_create(&r300->context);
+        if (r300->draw == NULL)
+            goto fail;
         /* Enable our renderer. */
         draw_set_rasterize_stage(r300->draw, r300_draw_stage(r300));
         /* Disable converting points/lines to triangles. */