[extractor/common] Fix crash in _report_ignoring_subs #2899

Job	Run time
Linter	25s
Select tests from inputs	1s
Run tests (ubuntu-20.04, 2.7, cpython, core, sh)	1m 15s
Run tests (ubuntu-20.04, 3.2, cpython, core, sh)	1m 9s
Run tests (ubuntu-20.04, 3.5, cpython, core, sh)	1m 8s
Run tests (ubuntu-20.04, 3.9, cpython, core, sh)	1m 11s
Run tests (ubuntu-20.04, 3.11, cpython, core, sh)	54s
Run tests (ubuntu-20.04, pypy-2.7, cpython, core, sh)	2m 30s
Run tests (ubuntu-20.04, pypy-3.6, cpython, core, sh)	2m 31s
Run tests (ubuntu-20.04, pypy-3.7, cpython, core, sh)	2m 52s
Run tests (windows-2019, 3.4, cpython, core, bat)	3m 21s
Run tests (windows-2019, 3.4, cpython, nodownload, bat)	55s
Run tests (ubuntu-20.04, 2.7, jython, core, sh)	3m 10s
Run tests (ubuntu-20.04, 2.7, jython, nodownload, sh)	30s
	21m 52s

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