[gnome-alsamixer] Fix use of undefined variable

[Bastien Nocera <hadess src gnome org>]

commit 151ed0bdac42fb7e9db618d261efa423ded1ca87
Author: Bastien Nocera <hadess hadess net>
Date:   Tue Jun 26 15:00:18 2012 +0100

    Fix use of undefined variable

 src/gam-slider.c |    3 ++-
 1 files changed, 2 insertions(+), 1 deletions(-)
---
diff --git a/src/gam-slider.c b/src/gam-slider.c
index 34ee7b7..68cf3a6 100644
--- a/src/gam-slider.c
+++ b/src/gam-slider.c
@@ -630,10 +630,11 @@ gam_slider_get_mixer (GamSlider *gam_slider)
     GamSliderPrivate *priv;
 
     g_return_val_if_fail (GAM_IS_SLIDER (gam_slider), NULL);
-    g_return_val_if_fail (GAM_IS_MIXER (priv->mixer), NULL);
 
     priv = GAM_SLIDER_GET_PRIVATE (gam_slider);
 
+    g_return_val_if_fail (GAM_IS_MIXER (priv->mixer), NULL);
+
     return priv->mixer;
 }