src/libadacl.c: fix a clang-tidy warning by adding a redundant check.

The clang-tidy program is complaining about a potential null pointer
dereference where none is possible. To convince it of that fact, I've
added a redundant equality check: we have a case where (a == b) and (a
== c), and I've added (b == c) explicitly. This fixes the warning, and
should have very little performance impact, so everyone is happy.