inotify_update: fix assertion failure when music dir is deleted

This shouldn't really happen, but insane users might delete/rename the
music directory while MPD runs.  What was even more insane was that
MPD crashed due to this.  This is a workaround - there is currently
nothing useful we can do in this case; except maybe poll for the music
directory to reappear, but that's too much trouble for a user error.

src/inotify_update.c

@@ -97,7 +97,13 @@ static void
 remove_watch_directory(struct watch_directory *directory)
 {
 	assert(directory != NULL);
-	assert(directory->parent != NULL);
+
+	if (directory->parent == NULL) {
+		g_warning("music directory was removed - "
+			  "cannot continue to watch it");
+		return;
+	}
+
 	assert(directory->parent->children != NULL);
 
 	tree_remove_watch_directory(directory);