ALSA: pcm: potential uninitialized return values

Smatch complains that "tmp" can be uninitialized if we do a zero size
write.

Fixes: 02a5d6925cd3 ("ALSA: pcm: Avoid potential races between OSS ioctls and read/write")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>

sound/core/oss/pcm_oss.c

@@ -1326,7 +1326,7 @@ static ssize_t snd_pcm_oss_write2(struct snd_pcm_substream *substream, const cha
 static ssize_t snd_pcm_oss_write1(struct snd_pcm_substream *substream, const char __user *buf, size_t bytes)
 {
 	size_t xfer = 0;
-	ssize_t tmp;
+	ssize_t tmp = 0;
 	struct snd_pcm_runtime *runtime = substream->runtime;
 
 	if (atomic_read(&substream->mmap_count))
@@ -1433,7 +1433,7 @@ static ssize_t snd_pcm_oss_read2(struct snd_pcm_substream *substream, char *buf,
 static ssize_t snd_pcm_oss_read1(struct snd_pcm_substream *substream, char __user *buf, size_t bytes)
 {
 	size_t xfer = 0;
-	ssize_t tmp;
+	ssize_t tmp = 0;
 	struct snd_pcm_runtime *runtime = substream->runtime;
 
 	if (atomic_read(&substream->mmap_count))