theme: Fix a memory leak

Commit 5c33b0d tried to copy/paste the code that constructed
classes, but for some reason didn't copy the theme name.

https://bugzilla.gnome.org/show_bug.cgi?id=690317

src/ui/theme.c

@@ -5467,6 +5467,8 @@ meta_theme_create_style_context (GdkScreen   *screen,
                                       GTK_STYLE_PROVIDER_PRIORITY_SETTINGS);
     }
 
+  g_free (theme_name);
+
   return style;
 }