Valgrind has started failing

[radarhere]

Valgrind is currently failing on main - https://github.com/python-pillow/Pillow/runs/7603531089?check_suite_focus=true#step:5:9512

Valgrind has detected an error:

**326** 
**326** **********************************************************************
**326** Tests/test_file_mpo.py::test_save_all
**326** **********************************************************************
==326== Use of uninitialised value of size 8
==326==    at 0x8A2B05B: ??? (in /usr/lib/x86_64-linux-gnu/libjpeg.so.8.2.2)
==326==    by 0x8A26F0D: ??? (in /usr/lib/x86_64-linux-gnu/libjpeg.so.8.2.2)
==326==    by 0x8A2D341: ??? (in /usr/lib/x86_64-linux-gnu/libjpeg.so.8.2.2)
==326==    by 0x8A25858: jpeg_read_scanlines (in /usr/lib/x86_64-linux-gnu/libjpeg.so.8.2.2)
==326==    by 0x89AB4A7: ImagingJpegDecode (JpegDecode.c:251)
==326==    by 0x897D8B9: _decode (decode.c:132)
==326==    by 0x64C9A5: method_vectorcall_VARARGS (descrobject.c:300)
==326==    by 0x4EBB06: _PyObject_Vectorcall (abstract.h:127)
==326==    by 0x4EBB06: call_function (ceval.c:4963)
==326==    by 0x4EBB06: _PyEval_EvalFrameDefault (ceval.c:3486)
==326==    by 0x4DF6B8: PyEval_EvalFrameEx (ceval.c:741)
==326==    by 0x4362AF: function_code_fastcall (call.c:284)
==326==    by 0x436D55: _PyFunction_Vectorcall (call.c:411)
==326==    by 0x645A86: _PyObject_Vectorcall (abstract.h:127)
==326==    by 0x645A86: method_vectorcall (classobject.c:60)
==326== 
**326** 
**326** **********************************************************************

The test was only recently added in #6444. However, that PR had no C changes, so Valgrind wasn't run.

If I revert #6444, then Valgrind passes - https://github.com/radarhere/Pillow/actions/runs/2772211965

[radarhere]

I've found that updating the Valgrind job to Jammy resolves this - https://github.com/radarhere/Pillow/runs/7622689068. So I've created #6473