llvmpipe: don't assume pipe_context is always available in flush_frontbuffer

Fixes: 91dcadf956 ("llvmpipe: finish rendering before flushing frontbuffer resources.")

Reviewed-by: Jose Fonseca <jfonseca@vmware.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/18392>

src/gallium/drivers/llvmpipe/lp_screen.c

@@ -838,7 +838,8 @@ llvmpipe_flush_frontbuffer(struct pipe_screen *_screen,
    assert(texture->dt);
 
    if (texture->dt) {
-      llvmpipe_flush_resource(_pipe, resource, 0, true, true, false, "frontbuffer");
+      if (_pipe)
+         llvmpipe_flush_resource(_pipe, resource, 0, true, true, false, "frontbuffer");
       winsys->displaytarget_display(winsys, texture->dt, context_private, sub_box);
    }
 }