Fix SEGV at unget to a null device StringIO #375

Job	Run time
ruby-versions / Generate Ruby versions	4s
windows-latest 2.7	1m 49s
windows-latest 3.0	1m 7s
windows-latest 3.1	1m 29s
windows-latest 3.2	1m 40s
windows-latest 3.3	1m 37s
windows-latest 3.4	3m 24s
windows-latest jruby-head	1m 51s
windows-latest mingw	1m 17s
windows-latest mswin	1m 12s
	15m 30s

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