MediaRoster: Don't allow rosters without a BApplication

* The BeBook states that any media_kit app should have BApplication
behind. Beware the app don't need to be running, but the object
should be present. This is because we use BApplication as a safe
exit point to free the memory allocated.
* While I was a bit reclutant in doing that, after a developer
discussion we agreed this would be the cleaner way to solve this
problem without eluding it.

src/kits/media/MediaRoster.cpp

@@ -2302,7 +2302,7 @@ BMediaRoster::Roster(status_t* out_error)
 	BAutolock lock(sInitLocker);
 
 	if (be_app == NULL)
-		TRACE("Warning! You should have a valid BApplication.");
+		debugger("Warning! You should have a valid BApplication.");
 
 	if (!lock.IsLocked())
 		return NULL;