nk_context commands buffer pollutes on skipping rendering.

Hi, I am having issues with optimizing render loop with `memcmp(cmds, last, cmds->allocated)`, if I am getting 0 then I call `nk_clear(ctx)` directly as documentation suggested.

But later at some point of rendering I found out I would have problems with `cmds`. I am getting garbage `cmd->type` and very big `cmd->next`, especially in the case of pop-up windows. It seems I need to call `nk_foreach` regardless I want to render or not before calling `nk_clear(ctx)`. I digged a bit and found that there was a `nk_build` function underneath that changes the command buffer.

I am not sure if this is the issue but I found calling a dummy `nk_foreach` is like a hack. Is there a better way doing it?

Start by tracing the command-buffer paths through nk_build, nk_foreach, and nk_clear, focusing on the skipped-rendering and pop-up cases described in the issue. Reproduce the garbage cmd->type and unusually large cmd->next values, then establish whether clearing without traversal is supported and what behavior should remain valid afterward.

Written by the indexing model from the issue text.