fix memory out of bound access and stack overflow in UTF8_validate

Fix directory location mismatches

Fix typo in printf format string

Signed-off-by: Dominik Thalhammer <dominik@thalhammer.it>

Signed-off-by: Otavio Rodolfo Piske <opiske@redhat.com>

Returns a human-redable error string.
Signed-off-by: David Wagner <david.wagner@easymile.com>

Secure environment variables are not available in PR builds

cf. https://docs.travis-ci.com/user/pull-requests/#Pull-Requests-and-Security-Restrictions

Therefore, only use them in official builds.
Signed-off-by: David Wagner <david.wagner@easymile.com>