[extractor/common] Fix crash in _report_ignoring_subs #2898

Job	Run time
Select tests from inputs	1s
Linter	31s
Run tests (ubuntu-20.04, 2.7, cpython, core, sh)	1m 8s
Run tests (ubuntu-20.04, 3.2, cpython, core, sh)	1m 10s
Run tests (ubuntu-20.04, 3.5, cpython, core, sh)	1m 17s
Run tests (ubuntu-20.04, 3.9, cpython, core, sh)	1m 11s
Run tests (ubuntu-20.04, 3.11, cpython, core, sh)	1m 12s
Run tests (ubuntu-20.04, pypy-2.7, cpython, core, sh)	2m 43s
Run tests (ubuntu-20.04, pypy-3.6, cpython, core, sh)	2m 40s
Run tests (ubuntu-20.04, pypy-3.7, cpython, core, sh)	2m 45s
Run tests (windows-2019, 3.4, cpython, core, bat)	4m 17s
Run tests (windows-2019, 3.4, cpython, nodownload, bat)	1m 13s
Run tests (ubuntu-20.04, 2.7, jython, core, sh)	3m 42s
Run tests (ubuntu-20.04, 2.7, jython, nodownload, sh)	26s
	24m 16s

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