prefs: Don't crash when we get a preference changed we don't know about

We commonly used the generic, undetailed signal 'changed' to track
changes to preferences. Since we crash on unknown preference types,
this can be dangerous if somebody adds a new setting that has a
type we're unfamiliar with, and something else changes it.

Instead of crashing, just fizzle out doing nothing.

src/core/prefs.c

@@ -1210,8 +1210,9 @@ settings_changed (GSettings *settings,
     }
   else
     {
-      /* Someone added a preference of an unhandled type */
+      /* Unknown preference type. This quite likely simply isn't
-      g_assert_not_reached ();
+       * a preference we track changes to. */
+      return;
     }
 
   g_variant_unref (value);