ALSA: ctxfi - Check the presence of SRC instance in PCM pointer callbacks

The SRC instances may not exist when PCM pointer callback is called at
the state before initialization is finished.  Add the NULL check just
to be sure.
Signed-off-by: NTakashi Iwai <tiwai@suse.de>

sound/pci/ctxfi/ctatc.c

@@ -445,6 +445,8 @@ atc_pcm_playback_position(struct ct_atc *atc, struct ct_atc_pcm *apcm)
 	u32 size, max_cisz;
 	int position;
 
+	if (!src)
+		return 0;
 	position = src->ops->get_ca(src);
 
 	size = apcm->vm_block->size;
@@ -782,6 +784,8 @@ atc_pcm_capture_position(struct ct_atc *atc, struct ct_atc_pcm *apcm)
 {
 	struct src *src = apcm->src;
 
+	if (!src)
+		return 0;
 	return src->ops->get_ca(src) - apcm->vm_block->addr;
 }