fix: prevent segfault in StringIO#seek with SEEK_END on null device (… #475

Job	Run time
ruby-versions / Generate Ruby versions	4s
ubuntu-24.04 3.0	21s
ubuntu-24.04 2.7	52s
ubuntu-24.04 jruby-head	1m 24s
ubuntu-24.04 truffleruby-head	41s
ubuntu-24.04 3.2	25s
ubuntu-22.04 3.0	21s
ubuntu-24.04 3.4	25s
ubuntu-22.04 3.2	22s
ubuntu-22.04 head	22s
ubuntu-24.04 debug	34s
ubuntu-22.04 3.4	28s
ubuntu-22.04 debug	29s
ubuntu-22.04 2.7	59s
ubuntu-22.04 truffleruby-head	39s
ubuntu-22.04 3.1	29s
ubuntu-22.04 jruby-head	1m 30s
ubuntu-24.04 3.3	23s
ubuntu-22.04 3.3	26s
ubuntu-24.04 3.1	22s
ubuntu-24.04 head	27s
	12m 3s

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