Its possible sstate symlinks to other sstate mirrors which then my get
removed/cleaned. If we find invalid symlinks, skip over them rather
than error with a backtrace.
(From OE-Core rev: 5ed9bb42abf93aa084dd23ca68cc996a94a51a10)
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>