ALSA: emu10k1: Fix possible NULL dereference

The previous dev_err() conversion resulted in a code that may give
NULL dereference in snd_emu10k1_ptr_write().  Since it's a sanity
check, better to be replaced with a debug macro like other places in
this driver.

Fixes: 6f002b02166c ('ALSA: emu10k1: Use standard printk helpers')
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>