[metacity] workspace: fix a build warning

[Alberts Muktupāvels <muktupavels src gnome org>]

commit 8888e890ce8d0b1ac22782ab234e8e7a71ac43c8
Author: Alberts Muktupāvels <alberts muktupavels gmail com>
Date:   Mon Jul 3 15:28:42 2017 +0300

    workspace: fix a build warning

 src/core/workspace.c |    3 +++
 1 files changed, 3 insertions(+), 0 deletions(-)
---
diff --git a/src/core/workspace.c b/src/core/workspace.c
index 051a2a2..328b8ee 100644
--- a/src/core/workspace.c
+++ b/src/core/workspace.c
@@ -347,6 +347,7 @@ static void workspace_switch_sound(MetaWorkspace *from,
   {
     const char *e;
 
+    e = NULL;
     if (x < layout.current_col)
       e = "desktop-switch-left";
     else if (x > layout.current_col)
@@ -355,6 +356,8 @@ static void workspace_switch_sound(MetaWorkspace *from,
       e = "desktop-switch-up";
     else if (y > layout.current_row)
       e = "desktop-switch-down";
+    else
+      g_assert_not_reached ();
 
     ca_context_play(ca_gtk_context_get(), 1,
                   CA_PROP_EVENT_ID, e,