[ALSA] Add error messages

Add error messages in the critial error path to be more verbose.

Signed-off-by: Takashi Iwai <tiwai@suse.de>
diff --git a/sound/core/control.c b/sound/core/control.c
index 03ae9bb..f8f98cc 100644
--- a/sound/core/control.c
+++ b/sound/core/control.c
@@ -195,8 +195,10 @@
 	snd_assert(control != NULL, return NULL);
 	snd_assert(control->count > 0, return NULL);
 	kctl = kzalloc(sizeof(*kctl) + sizeof(struct snd_kcontrol_volatile) * control->count, GFP_KERNEL);
-	if (kctl == NULL)
+	if (kctl == NULL) {
+		snd_printk(KERN_ERR "Cannot allocate control instance\n");
 		return NULL;
+	}
 	*kctl = *control;
 	for (idx = 0; idx < kctl->count; idx++)
 		kctl->vd[idx].access = access;
@@ -309,7 +311,9 @@
 	struct snd_ctl_elem_id id;
 	unsigned int idx;
 
-	snd_assert(card != NULL && kcontrol != NULL, return -EINVAL);
+	snd_assert(card != NULL, return -EINVAL);
+	if (! kcontrol)
+		return -EINVAL;
 	snd_assert(kcontrol->info != NULL, return -EINVAL);
 	id = kcontrol->id;
 	down_write(&card->controls_rwsem);