Second part of fix for bug 274819 -- g-s-d crashes when default sink changes

Taken from upstream git repository

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From 5b73218d9170626162fca03425ac7f0b4789bc0f Mon Sep 17 00:00:00 2001
From: Chris Coulson <chrisccoulson@googlemail.com>
Date: Fri, 31 Jul 2009 15:59:36 +0000
Subject: Bug 590073 – gnome-settings-daemon crashed with SIGSEGV in gvc_mixer_stream_is_running()

Fix crasher when the default sink changes, we were unref'ing
streams, when never ref'ing them.
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diff --git a/plugins/media-keys/gsd-media-keys-manager.c b/plugins/media-keys/gsd-media-keys-manager.c
index 3eeea71..8438875 100644
--- a/plugins/media-keys/gsd-media-keys-manager.c
+++ b/plugins/media-keys/gsd-media-keys-manager.c
@@ -740,7 +740,7 @@ update_default_sink (GsdMediaKeysManager *manager)
         }
 
         if (stream != NULL) {
-                manager->priv->stream = stream;
+                manager->priv->stream = g_object_ref (stream);
                 g_signal_connect (G_OBJECT (manager->priv->stream), "notify::volume",
                                   G_CALLBACK (on_stream_event_notify), manager);
                 g_signal_connect (G_OBJECT (manager->priv->stream), "notify::is-muted",
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