ALSA: pcm core - add a safe check to the silence filling function

In situation when appl_ptr is far greater then hw_ptr, the hw_avail value
can be greater than buffer_size. Check for this.
Signed-off-by: Jaroslav Kysela <perex@perex.cz>

sound/core/pcm_lib.c

@@ -67,6 +67,8 @@ void snd_pcm_playback_silence(struct snd_pcm_substream *substream, snd_pcm_ufram
 	} else {
 		if (new_hw_ptr == ULONG_MAX) {	/* initialization */
 			snd_pcm_sframes_t avail = snd_pcm_playback_hw_avail(runtime);
+			if (avail > runtime->buffer_size)
+				avail = runtime->buffer_size;
 			runtime->silence_filled = avail > 0 ? avail : 0;
 			runtime->silence_start = (runtime->status->hw_ptr +
 						  runtime->silence_filled) %