ostream: fix potential overwrite in StrBuf::sync_impl #1600

Job	Run time
init (ubuntu-22.04)	8m 47s
init (ubuntu-24.04)	11m 42s
Code QL Analyze (ubuntu-22.04)	6m 56s
Build using autoconf tarball (ubuntu-22.04)	4m 9s
Code QL Analyze (ubuntu-24.04)	7m 4s
cognitive-complexity	13s
Build using autoconf tarball (ubuntu-24.04)	4m 17s
compile (ubuntu-22.04)	2m 28s
compile (ubuntu-22.04, -Dbuildtype=debug -Db_lto=false)	1m 53s
scan-build	14s
compile (ubuntu-22.04, -Dbuildtype=debug -Db_lto=false -Dplugin-syscalls=false)	1m 53s
compile (ubuntu-22.04, -Dbuildtype=debug -Db_lto=false -Db_sanitize=address,undefined)	2m 47s
compile (ubuntu-22.04, -Dbuildtype=debug -Db_lto=false -Drepl=true)	2m 9s
compile (ubuntu-22.04, -Dbuildtype=debug -Db_lto=false -Dthreadsafety=true)	1m 55s
compile (ubuntu-24.04)	2m 40s
compile (ubuntu-24.04, -Dbuildtype=debug -Db_lto=false)	1m 57s
compile (ubuntu-24.04, -Dbuildtype=debug -Db_lto=false -Dplugin-syscalls=false)	1m 50s
compile (ubuntu-24.04, -Dbuildtype=debug -Db_lto=false -Db_sanitize=address,undefined)	2m 22s
compile (ubuntu-24.04, -Dbuildtype=debug -Db_lto=false -Drepl=true)	1m 56s
compile (ubuntu-24.04, -Dbuildtype=debug -Db_lto=false -Dthreadsafety=true)	1m 58s
	1h 9m 10s

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