So where are these non-NULL checks in the new code?

At the very beginning of dsp_close_chan().

Are the values of priv->{rd,wr}ch stable? You are now checking for their non-NULL after priv->sc acquire.

Primary (i.e., non-vchan) channels are destroyed during detach (see pcm_killchans()), and vchans in dsp_close() (see vchan_destroy()). So I think it's safe to leave the check as is. If we're detaching, we'll catch that a few lines below and bail-out anyway, and for vchans, well, we destroy them here only.