ALSA: gus: fix null pointer dereference on pointer block

The pointer block return from snd_gf1_dma_next_block could be
null, so there is a potential null pointer dereference issue.
Fix this by adding a null check before dereference.

Signed-off-by: Chengfeng Ye <cyeaa@connect.ust.hk>
Link: https://lore.kernel.org/r/20211024104611.9919-1-cyeaa@connect.ust.hk


Signed-off-by: Takashi Iwai <tiwai@suse.de>