libsmack: fixed segfault in get_label() (regression in 66483b)

Commit 66483b introduced segfault in API functions smack_revoke_subject()
and smack_set_label_for_self().
Fix the accidental NULL pointer dereference.
libsmack: parse whole access type string, not only first 5 bytes.

Previous version of this function parsed only first ACC_LEN (5)
characters of access_type. Now the whole string will be read.

This will prevent silent ignoring of access type characters in cases
like "-rwxat" or "rrwxat".

Re-applying because it was reverted by d2283792.
Merge remote-tracking branch 'rafal-krypa/issue68' into v1.0.x