Replace an assertion with a check-and-log

[nmathewson]

We hit this assertion with bug 32868, but I'm stymied figuring out
how we wound up with a routerstatus like this. This patch is a
diagnostic to attempt to figure out what is going on, and to avoid a
crash in the meantime.

[coveralls]

Pull Request Test Coverage Report for Build 7772

• 1 of 20 (5.0%) changed or added relevant lines in 1 file are covered.
• 13 unchanged lines in 2 files lost coverage.
• Overall coverage increased (+1.7%) to 60.864%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/feature/nodelist/node_select.c	1	20	5.0%

Files with Coverage Reduction	New Missed Lines	%
src/feature/dirauth/shared_random.c	4	85.1%
src/feature/dirauth/dirvote.c	9	64.65%

Totals
Change from base Build 7615:	1.7%
Covered Lines:	44551
Relevant Lines:	73198

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💛 - Coveralls