Fix NULL pointer access (crashes the sim on windows) in directories with no displayed entries.

git-svn-id: svn://svn.rockbox.org/rockbox/trunk@13059 a1c6a512-1295-4272-9138-f99709370657

apps/tree.c

@@ -488,7 +488,7 @@ void get_current_file(char* buffer, int buffer_len)
     struct entry* dc = tc.dircache;
     struct entry* e = &dc[tc.selected_item];
     snprintf(buffer, buffer_len, "%s/%s", getcwd(NULL,0),
-                                          e->name);
+                                          tc.dirlength ? e->name : "");
 }
 
 /* Selects a file and update tree context properly */