renderer: avoid crash in vrend_destroy_shader_selector()

If we didn't run succesfully vrend_destroy_shader_selector(),
sel->sinfo.so_names might be NULL.

Fix found thanks to american fuzzy lop.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>

src/vrend_renderer.c

@@ -560,8 +560,9 @@ static void vrend_destroy_shader_selector(struct vrend_shader_selector *sel)
       vrend_shader_destroy(p);
       p = c;
    }
-   for (i = 0; i < sel->sinfo.so_info.num_outputs; i++)
+   if (sel->sinfo.so_names)
-      free(sel->sinfo.so_names[i]);
+      for (i = 0; i < sel->sinfo.so_info.num_outputs; i++)
+         free(sel->sinfo.so_names[i]);
    free(sel->tmp_buf);
    free(sel->sinfo.so_names);
    free(sel->sinfo.interpinfo);