The addition of some debugging code meant that comparisions between sig
files with a taint and without a taint weren't working. Tweak the logic
to avoid tracebacks if one side doesn't have a taint.
(Bitbake rev: f5ea06fc2b6713c9f8e85ecf7cb981ae9a84d896)
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>