[extractor/common] Fix crash in _report_ignoring_subs #2894

Job	Run time
Linter	18s
Select tests from inputs	1s
Run tests (ubuntu-20.04, 2.7, cpython, core, sh)	1m 5s
Run tests (ubuntu-20.04, 3.2, cpython, core, sh)	1m 5s
Run tests (ubuntu-20.04, 3.5, cpython, core, sh)	1m 7s
Run tests (ubuntu-20.04, 3.9, cpython, core, sh)	1m 9s
Run tests (ubuntu-20.04, 3.11, cpython, core, sh)	1m 10s
Run tests (ubuntu-20.04, pypy-2.7, cpython, core, sh)	2m 29s
Run tests (ubuntu-20.04, pypy-3.6, cpython, core, sh)	2m 57s
Run tests (ubuntu-20.04, pypy-3.7, cpython, core, sh)	2m 35s
Run tests (windows-2019, 3.4, cpython, core, bat)	2m 0s
Run tests (windows-2019, 3.4, cpython, nodownload, bat)	1m 5s
Run tests (ubuntu-20.04, 2.7, jython, core, sh)	3m 12s
Run tests (ubuntu-20.04, 2.7, jython, nodownload, sh)	24s
	20m 37s

Provide feedback