Do not set "once" when calling tor_bug_occurred_ from BUG().

[nmathewson]

The "once" flag makes tor_bug_occurred_() say that future instances
of the warning will be suppressed -- but that's not something that
BUG() does. Fixes bug 33095; bugfix on 0.4.1.1-alpha.

[coveralls]

Pull Request Test Coverage Report for Build 7921

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 13 unchanged lines in 2 files lost coverage.
• Overall coverage decreased (-0.02%) to 61.554%

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Files with Coverage Reduction	New Missed Lines	%
src/feature/dirauth/shared_random.c	4	85.0%
src/feature/dirauth/dirvote.c	9	64.61%

Totals
Change from base Build 7914:	-0.02%
Covered Lines:	47458
Relevant Lines:	77100

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💛 - Coveralls