V4L/DVB (8026): Avoids an OOPS if dev struct can't be successfully recovered

On some alsa versions, it seems that snd_pcm_substream_chip(substream)
is returning a NULL pointer. This causes an OOPS, as reported by:

https://bugs.launchpad.net/ubuntu/+source/linux-ubuntu-modules-2.6.24/+bug/212271
https://bugs.launchpad.net/ubuntu/+source/alsa-driver/+bug/212960

This patch avoids the OOPS by not letting and open() succeed.
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>

drivers/media/video/cx88/cx88-alsa.c

@@ -332,6 +332,12 @@ static int snd_cx88_pcm_open(struct snd_pcm_substream *substream)
 	struct snd_pcm_runtime *runtime = substream->runtime;
 	int err;
 
+	if (!chip) {
+		printk(KERN_ERR "BUG: cx88 can't find device struct."
+				" Can't proceed with open\n");
+		return -ENODEV;
+	}
+
 	err = snd_pcm_hw_constraint_pow2(runtime, 0, SNDRV_PCM_HW_PARAM_PERIODS);
 	if (err < 0)
 		goto _error;

drivers/media/video/em28xx/em28xx-audio.c

@@ -268,6 +268,12 @@ static int snd_em28xx_capture_open(struct snd_pcm_substream *substream)
 
 	dprintk("opening device and trying to acquire exclusive lock\n");
 
+	if (!dev) {
+		printk(KERN_ERR "BUG: em28xx can't find device struct."
+				" Can't proceed with open\n");
+		return -ENODEV;
+	}
+
 	/* Sets volume, mute, etc */
 
 	dev->mute = 0;

drivers/media/video/saa7134/saa7134-alsa.c

@@ -613,9 +613,15 @@ static int snd_card_saa7134_capture_open(struct snd_pcm_substream * substream)
 	struct snd_pcm_runtime *runtime = substream->runtime;
 	snd_card_saa7134_pcm_t *pcm;
 	snd_card_saa7134_t *saa7134 = snd_pcm_substream_chip(substream);
-	struct saa7134_dev *dev = saa7134->dev;
+	struct saa7134_dev *dev;
 	int amux, err;
 
+	if (!saa7134) {
+		printk(KERN_ERR "BUG: saa7134 can't find device struct."
+				" Can't proceed with open\n");
+		return -ENODEV;
+	}
+	dev = saa7134->dev;
 	mutex_lock(&dev->dmasound.lock);
 
 	dev->dmasound.read_count  = 0;