devel_themer_cleanup() deletes other users' files

devel_themer_cleanup() doesn't check for ownership on temp files before attempting to delete them. On shared hosting, if any two sites run Devel without setting upload_tmp_dir(), this will generate E_WARNING messages ('unlink(/tmp/devel_themer_14019572574a8f2d12e01b0) [function.unlink]: Operation not permitted') as it attempts to delete the wrong users' files.