[Bug_Fix] shortcut to directory in .link files caused crash

since the plugin browser is now closed when running plugins
rb->set_current_file() had no valid browser context and used 'random' memory
instead

also adds a way to discard levels so we can load the desired directory
instead of returning to the previous

https://forums.rockbox.org/index.php/topic,54694.0.html

Change-Id: I624246e56d42972bf6a1ce566a209b745de6f30b

apps/open_plugin.c

@@ -353,7 +353,7 @@ static bool callback_show_item(char *name, int attr, struct tree_context *tc)
     {
         if (strstr(tc->currdir, PLUGIN_DIR) != NULL)
             return true;
-        tc->browse = NULL; /* exit immediately */
+        tc->is_browsing = false; /* exit immediately */
     }
     else if(attr & FILE_ATTR_ROCK)
     {