Stop logging a BUG() warning when tor is waiting for exit descriptors

[nmathewson]

Fixes bug 28656; bugfix on 0.3.5.1-alpha.

[coveralls]

Pull Request Test Coverage Report for Build 4249

• 0 of 1 (0.0%) changed or added relevant line in 1 file are covered.
• 92 unchanged lines in 8 files lost coverage.
• Overall coverage decreased (-0.1%) to 60.302%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/feature/nodelist/nodelist.c	0	1	0.0%

Files with Coverage Reduction	New Missed Lines	%
src/feature/nodelist/nodelist.c	2	61.35%
src/lib/container/buffers.c	2	90.93%
src/lib/fs/files.c	2	67.21%
src/feature/dirauth/shared_random.c	3	85.64%
src/feature/dirauth/dirvote.c	9	64.79%
src/lib/log/log.c	10	72.14%
src/lib/net/socket.c	10	46.25%
src/lib/net/buffers_net.c	54	0.0%

Totals
Change from base Build 4222:	-0.1%
Covered Lines:	42854
Relevant Lines:	71066

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💛 - Coveralls