Fix FS#5797 (Codec failure with directory skip when in file browser, with dircache...

Fix FS#5797 (Codec failure with directory skip when in file browser, with dircache enabled). Marking the tree context to be reloaded too early would cause the new playlist to contain files that don't exist (correct directory but wrong filenames), in turn causing the codec failures. The fix is to call reload_directory() after the playlist has been created.
I also added a sanity check to avoid loading an unknown codec, which was happening as a consequence of this issue.

git-svn-id: svn://svn.rockbox.org/rockbox/trunk@16039 a1c6a512-1295-4272-9138-f99709370657