[extractor/common] Fix crash in _report_ignoring_subs #2897

Job	Run time	Billable time
Linter	16s	0s
Select tests from inputs	1s	0s
Run tests (ubuntu-20.04, 2.7, cpython, core, sh)	1m 10s	0s
Run tests (ubuntu-20.04, 3.2, cpython, core, sh)	1m 11s	0s
Run tests (ubuntu-20.04, 3.5, cpython, core, sh)	1m 18s	0s
Run tests (ubuntu-20.04, 3.9, cpython, core, sh)	1m 12s	0s
Run tests (ubuntu-20.04, 3.11, cpython, core, sh)	1m 7s	0s
Run tests (ubuntu-20.04, pypy-2.7, cpython, core, sh)	2m 35s	0s
Run tests (ubuntu-20.04, pypy-3.6, cpython, core, sh)	2m 39s	0s
Run tests (ubuntu-20.04, pypy-3.7, cpython, core, sh)	2m 52s	0s
Run tests (windows-2019, 3.4, cpython, core, bat)	2m 25s	0s
Run tests (windows-2019, 3.4, cpython, nodownload, bat)	1m 8s	0s
Run tests (ubuntu-20.04, 2.7, jython, core, sh)	3m 7s	0s
Run tests (ubuntu-20.04, 2.7, jython, nodownload, sh)	22s	0s
	21m 23s	0s

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