Fix regression in do_write(s) causing significant performance issues when using large (>10meg) writes #919

Job	Run time
openssl-1.0.2u	2m 24s
openssl-1.1.0l	1m 29s
openssl-1.1.1w	1m 45s
openssl-3.0.12	2m 52s
openssl-3.1.4	2m 56s
openssl-3.2.0	3m 31s
libressl-3.1.5	2m 7s
libressl-3.2.7	2m 12s
libressl-3.3.6	2m 12s
libressl-3.4.3	2m 14s
libressl-3.5.3	2m 15s
libressl-3.6.3	2m 13s
libressl-3.7.3	2m 17s
libressl-3.8.1	2m 10s
openssl-3.0.12 fips	2m 48s
openssl-3.1.4 fips	2m 41s
openssl-3.2.0 fips	3m 22s
openssl-head	3m 22s
openssl-head fips	3m 21s
ruby-versions / Generate Ruby versions	2s
ubuntu-22.04 2.7	58s
ubuntu-22.04 3.0	54s
ubuntu-22.04 3.1	1m 2s
ubuntu-22.04 3.2	57s
ubuntu-22.04 3.3	54s
ubuntu-22.04 head	55s
ubuntu-22.04 truffleruby	1m 31s
ubuntu-22.04 truffleruby-head	1m 33s
ubuntu-20.04 2.7	54s
ubuntu-20.04 3.0	52s
ubuntu-20.04 3.1	51s
ubuntu-20.04 3.2	51s
ubuntu-20.04 3.3	50s
ubuntu-20.04 head	52s
ubuntu-20.04 truffleruby-head	1m 31s
macos-latest 2.7	1m 30s
macos-latest 3.0	1m 18s
macos-latest 3.1	1m 36s
macos-latest 3.2	1m 3s
macos-latest 3.3	1s
macos-latest head	54s
macos-latest truffleruby-head	1m 50s
windows-latest 2.7	4m 49s
windows-latest 3.0	4m 27s
windows-latest 3.1	4m 37s
windows-latest 3.2	5m 1s
windows-latest 3.3	5m 33s
windows-latest ucrt	3m 55s
windows-latest mswin	2m 39s
	1h 42m 51s

Provide feedback