draw: fix no position output in non-llvm pipeline.

It seems easiest (and best) if we simply skip all the later stages
(after stream output).
(This is different to the llvm case at least for now where we will
simply try to render garbage, though both behaviors should be correct.)
Fixes piglit glsl-1.40-tf-no-position with softpipe.

Reviewed-by: Brian Paul <brianp@vmware.com>

src/gallium/auxiliary/draw/draw_pt_fetch_shade_pipeline.c

@@ -284,27 +284,27 @@ static void fetch_pipeline_generic( struct draw_pt_middle_end *middle,
     * XXX: Stream output surely needs to respect the prim_info->elt
     *      lists.
     */
-   draw_pt_so_emit( fpme->so_emit,
-                    vert_info,
-                    prim_info );
+   draw_pt_so_emit( fpme->so_emit, vert_info, prim_info );
 
-   if (draw_pt_post_vs_run( fpme->post_vs,
-                            vert_info ))
-   {
-      opt |= PT_PIPELINE;
-   }
-
-   /* Do we need to run the pipeline?
+   /*
+    * if there's no position, need to stop now, or the latter stages
+    * will try to access non-existent position output.
     */
-   if (opt & PT_PIPELINE) {
-      pipeline( fpme,
-                vert_info,
-                prim_info );
-   }
-   else {
-      emit( fpme->emit,
-            vert_info,
-            prim_info );
+   if (draw_current_shader_position_output(draw) != -1) {
+
+      if (draw_pt_post_vs_run( fpme->post_vs, vert_info ))
+      {
+         opt |= PT_PIPELINE;
+      }
+
+      /* Do we need to run the pipeline?
+       */
+      if (opt & PT_PIPELINE) {
+         pipeline( fpme, vert_info, prim_info );
+      }
+      else {
+         emit( fpme->emit, vert_info, prim_info );
+      }
    }
    FREE(vert_info->verts);
 }