selinux: remove redundant assignment to str

str is being assigned to an empty string but str is never being
read after that, so the assignment is redundant and can be removed.
Moving the declaration of str to a more localised block, cleans up
clang warning: "Value stored to 'str' is never read"

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Paul Moore <paul@paul-moore.com>