ALSA: memalloc: Catch call with NULL snd_dma_buffer pointer

ALSA: memalloc: Catch call with NULL snd_dma_buffer pointer

Although we've covered all calls with NULL dma buffer pointer, so far,
there may be still some else in the wild.  For catching such a case
more easily, add a WARN_ON_ONCE() in snd_dma_get_ops().

Fixes: 5665d92abd1b ("ALSA: core: Abstract memory alloc helpers")
Link: https://lore.kernel.org/r/20211105102103.28148-1-tiwai@suse.de
Signed-off-by: Takashi Iwai <tiwai@suse.de>