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

[nmathewson]

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.

[coveralls]

Pull Request Test Coverage Report for Build 4986

• 2 of 2 (100.0%) changed or added relevant lines in 1 file are covered.
• 19 unchanged lines in 4 files lost coverage.
• Overall coverage decreased (-0.02%) to 62.415%

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Files with Coverage Reduction	New Missed Lines	%
src/core/or/circuitpadding.c	3	92.07%
src/feature/dirauth/shared_random.c	3	85.64%
src/feature/hs/hs_service.c	4	73.63%
src/feature/dirauth/dirvote.c	9	64.74%

Totals
Change from base Build 4985:	-0.02%
Covered Lines:	46502
Relevant Lines:	74505

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💛 - Coveralls