frame->visited is used uninitialized in mark_stacks #148144

SUMMARY: MemorySanitizer: use-of-uninitialized-value (/mnt/scratch0/clusterfuzz/bot/builds/clusterfuzz-builds_python3-libraries_95a4264029708fc880f29203e678fdb3bff2689b/revisions/fuzzer-difflib+0xd28a39)
    ==251==WARNING: MemorySanitizer: use-of-uninitialized-value
        #0 0x5cccd3181a39 in mark_stacks cpython/Python/gc.c:1661:24
        #1 0x5cccd3164405 in gc_collect_increment cpython/Python/gc.c:1769:31
        #2 0x5cccd3164405 in _PyGC_Collect cpython/Python/gc.c:2191:13
        #3 0x5cccd31aecf6 in _Py_HandlePending cpython/Python/ceval_gil.c:1399:9
        #4 0x5cccd2fbf509 in check_periodics cpython/Python/ceval_macros.h:512:16
        #5 0x5cccd2fbf509 in _PyEval_EvalFrameDefault cpython/Python/generated_cases.c.h:7900:27
        #6 0x5cccd37954da in _PyEval_EvalFrame cpython/Include/internal/pycore_ceval.h:118:16
        #7 0x5cccd37954da in gen_send_ex2 cpython/Objects/genobject.c:280:24
        #8 0x5cccd37954da in gen_send_ex cpython/Objects/genobject.c:373:12
        #9 0x5cccd378b338 in gen_iternext cpython/Objects/genobject.c:764:9
        #10 0x5cccd2f9fe70 in builtin_next cpython/Python/bltinmodule.c:1764:11
        #11 0x5cccd2fb4b93 in _Py_BuiltinCallFast_StackRefSteal cpython/Python/ceval.c:825:11
        #12 0x5cccd2fe7878 in _PyEval_EvalFrameDefault cpython/Python/generated_cases.c.h:2378:35
        #13 0x5cccd2fb297f in _PyEval_EvalFrame cpython/Include/internal/pycore_ceval.h:118:16
        #14 0x5cccd2fb297f in _PyEval_Vector cpython/Python/ceval.c:2174:12