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: 6f002b0 ('ALSA: emu10k1: Use standard printk helpers')
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>

sound/pci/emu10k1/io.c

@@ -71,11 +71,8 @@ void snd_emu10k1_ptr_write(struct snd_emu10k1 *emu, unsigned int reg, unsigned i
 	unsigned long flags;
 	unsigned int mask;
 
-	if (!emu) {
-		dev_err(emu->card->dev, "ptr_write: emu is null!\n");
-		dump_stack();
+	if (snd_BUG_ON(!emu))
 		return;
-	}
 	mask = emu->audigy ? A_PTR_ADDRESS_MASK : PTR_ADDRESS_MASK;
 	regptr = ((reg << 16) & mask) | (chn & PTR_CHANNELNUM_MASK);