ALSA: pcm: Test for "silence" field in struct "pcm_format_data"

Syzbot reports "KASAN: null-ptr-deref Write in
snd_pcm_format_set_silence".[1]

It is due to missing validation of the "silence" field of struct
"pcm_format_data" in "pcm_formats" array.

Add a test for valid "pat" and, if it is not so, return -EINVAL.

[1] https://lore.kernel.org/lkml/000000000000d188ef05dc2c7279@google.com/

Reported-and-tested-by: syzbot+205eb15961852c2c5974@syzkaller.appspotmail.com
Signed-off-by: Fabio M. De Francesco <fmdefrancesco@gmail.com>
Cc: <stable@vger.kernel.org>
Link: https://lore.kernel.org/r/20220409012655.9399-1-fmdefrancesco@gmail.comSigned-off-by: Takashi Iwai <tiwai@suse.de>

sound/core/pcm_misc.c

@@ -433,7 +433,7 @@ int snd_pcm_format_set_silence(snd_pcm_format_t format, void *data, unsigned int
 		return 0;
 	width = pcm_formats[(INT)format].phys; /* physical width */
 	pat = pcm_formats[(INT)format].silence;
-	if (! width)
+	if (!width || !pat)
 		return -EINVAL;
 	/* signed or 1 byte data */
 	if (pcm_formats[(INT)format].signd == 1 || width <= 8) {