Avoid timeout of retry state

[ganmacs]

Which issue(s) this PR fixes:
Fixes https://travis-ci.org/github/fluent/fluentd/jobs/664260115#L1720

What this PR does / why we need it:

If it exceeds the retry limit, https://travis-ci.org/github/fluent/fluentd/jobs/664260115#L1720 can happen (setting nil to @retry).

fluentd/lib/fluent/plugin/output.rb

Line 1241 in b18f995

if @retry.limit?

if i become over 10, @i.retry can be nil.

fluentd/test/plugin/test_output_as_buffered_secondary.rb

Lines 619 to 625 in b18f995

	30.times do |i| # large enough
	now = first_failure + 60 * 0.8 + 2 + i
	Timecop.freeze( now )
	@i.flush_thread_wakeup
	break if @i.buffer.dequeued.size == 0
	end

Docs Changes:

no need

Release Note:

no need

[cosmo0920]

Good catch! I'd also investigated how to fix this failing test case, but I didn't find it....