USB: gadget: audio driver seg-fault fix

The included patch can be applied to the new usb gadget audio driver.

It addresses a seg-fault in uncovered in g_audio.ko.
The fault occurs in the function u_audio.c::gaudio_open_end_dev() when
device /dev/snd/pcmC0D0c (FILE_PCM_CAPTURE) is not present.

I suspect there may be similar problems with device /dev/snd/pcmC0D0p
(FILE_PCM_PLAYBACK) handling also.  I leave that for the developer(s),
as I was unsure as to the side-effects of not calling
playback_default_hw_params() in the initialization phase.
Signed-off-by: Robin Callender <robin_callender@hotmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

drivers/usb/gadget/u_audio.c

@@ -253,11 +253,13 @@ static int gaudio_open_snd_dev(struct gaudio *card)
 	snd->filp = filp_open(fn_cap, O_RDONLY, 0);
 	if (IS_ERR(snd->filp)) {
 		ERROR(card, "No such PCM capture device: %s\n", fn_cap);
-		snd->filp = NULL;
-	}
+		snd->substream = NULL;
+		snd->card = NULL;
+	} else {
 		pcm_file = snd->filp->private_data;
 		snd->substream = pcm_file->substream;
 		snd->card = card;
+	}
 
 	return 0;
 }