Don't pass a NULL into a %s when logging client auth file load failure

Fortunately, in 0.3.5.1-alpha we improved logging for various
failure cases involved with onion service client auth.

Unfortunately, for this one, we freed the file right before logging
its name.

Fortunately, tor_free() sets its pointer to NULL, so we didn't have
a use-after-free bug.

Unfortunately, passing NULL to %s is not defined.

Fortunately, GCC 9.1.1 caught the issue!

Unfortunately, nobody has actually tried building Tor with GCC 9.1.1
before. Or if they had, they didn't report the warning.

Fixes bug 30475; bugfix on 0.3.5.1-alpha.