Fix SEGV at unget to a null device StringIO #434

Job	Run time
ruby-versions / Generate Ruby versions	2s
ubuntu-24.04 2.7	41s
ubuntu-24.04 3.0	10s
ubuntu-24.04 3.1	14s
ubuntu-24.04 3.2	14s
ubuntu-24.04 3.3	15s
ubuntu-24.04 3.4	10s
ubuntu-24.04 debug	22s
ubuntu-24.04 head	15s
ubuntu-24.04 jruby-head	43s
ubuntu-24.04 truffleruby-head	25s
ubuntu-22.04 2.7	36s
ubuntu-22.04 3.0	10s
ubuntu-22.04 3.1	13s
ubuntu-22.04 3.2	21s
ubuntu-22.04 3.3	22s
ubuntu-22.04 3.4	11s
ubuntu-22.04 debug	21s
ubuntu-22.04 head	11s
ubuntu-22.04 jruby-head	51s
ubuntu-22.04 truffleruby-head	32s
	7m 19s

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