[ALSA] fix potential NULL pointer deref in snd_sb8dsp_midi_interrupt()

First testing if a pointer is NULL and if it is (or might be), proceeding
with code that dereferences that same pointer is clearly a mistake.
This happens in sound/isa/sb/sb8_midi.c::snd_sb8dsp_midi_interrupt()
The patch below reworks the code so this unfortunate case doesn't happen.
Also remove some blank comments.
Found by the Coverity checker as bug #367
Patch is compile testted only due to lack of hardware.

Signed-off-by: Jesper Juhl <jesper.juhl@gmail.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>

sound/isa/sb/sb8_midi.c

@@ -32,20 +32,22 @@
 #include <sound/core.h>
 #include <sound/sb.h>
 
-/*
-
- */
 
-irqreturn_t snd_sb8dsp_midi_interrupt(struct snd_sb * chip)
+irqreturn_t snd_sb8dsp_midi_interrupt(struct snd_sb *chip)
 {
 	struct snd_rawmidi *rmidi;
 	int max = 64;
 	char byte;
 
-	if (chip == NULL || (rmidi = chip->rmidi) == NULL) {
+	if (!chip)
+		return IRQ_NONE;
+	
+	rmidi = chip->rmidi;
+	if (!rmidi) {
 		inb(SBP(chip, DATA_AVAIL));	/* ack interrupt */
 		return IRQ_NONE;
 	}
+
 	spin_lock(&chip->midi_input_lock);
 	while (max-- > 0) {
 		if (inb(SBP(chip, DATA_AVAIL)) & 0x80) {
@@ -59,10 +61,6 @@ irqreturn_t snd_sb8dsp_midi_interrupt(struct snd_sb * chip)
 	return IRQ_HANDLED;
 }
 
-/*
-
- */
-
 static int snd_sb8dsp_midi_input_open(struct snd_rawmidi_substream *substream)
 {
 	unsigned long flags;
@@ -252,10 +250,6 @@ static void snd_sb8dsp_midi_output_trigger(struct snd_rawmidi_substream *substre
 		snd_sb8dsp_midi_output_write(substream);
 }
 
-/*
-
- */
-
 static struct snd_rawmidi_ops snd_sb8dsp_midi_output =
 {
 	.open =		snd_sb8dsp_midi_output_open,