fix: prevent segfault in StringIO#seek with SEEK_END on null device #473

Job	Run time
ruby-versions / Generate Ruby versions	3s
ubuntu-24.04 2.7	50s
ubuntu-24.04 3.1	28s
ubuntu-24.04 3.0	26s
ubuntu-24.04 3.2	27s
ubuntu-24.04 jruby-head	1m 16s
ubuntu-22.04 3.2	25s
ubuntu-22.04 2.7	54s
ubuntu-22.04 3.4	24s
ubuntu-22.04 3.3	6h 0m 3s
ubuntu-22.04 3.0	21s
ubuntu-22.04 truffleruby-head	35s
ubuntu-22.04 jruby-head	1m 15s
ubuntu-22.04 head	26s
ubuntu-22.04 debug	35s
ubuntu-24.04 head	21s
ubuntu-24.04 truffleruby-head	40s
ubuntu-22.04 3.1	24s
ubuntu-24.04 debug	31s
ubuntu-24.04 3.4	27s
ubuntu-24.04 3.3	25s
	6h 11m 16s

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