read: don't clear buffer when nothing can be read #1003

Job	Run time
openssl-1.0.2u	3m 3s
openssl-1.1.0l	1m 46s
openssl-1.1.1w	1m 44s
openssl-3.0.13	2m 55s
openssl-3.1.5	2m 56s
openssl-3.2.1	3m 20s
libressl-3.1.5	2m 5s
libressl-3.2.7	2m 24s
libressl-3.3.6	2m 13s
libressl-3.4.3	3m 20s
libressl-3.5.3	2m 17s
libressl-3.6.3	2m 25s
libressl-3.7.3	2m 16s
libressl-3.8.2	2m 23s
openssl-3.0.13 fips	2m 56s
openssl-3.1.5 fips	2m 58s
openssl-3.2.1 fips	3m 12s
openssl-head	3m 26s
openssl-head fips	3m 18s
openssl-head no-legacy	3m 38s
ruby-versions / Generate Ruby versions	3s
ubuntu-22.04 2.7	58s
ubuntu-22.04 3.0	53s
ubuntu-22.04 3.1	59s
ubuntu-22.04 3.2	58s
ubuntu-22.04 3.3	53s
ubuntu-22.04 head	54s
ubuntu-22.04 truffleruby	1m 27s
ubuntu-22.04 truffleruby-head	1m 39s
ubuntu-20.04 2.7	55s
ubuntu-20.04 3.0	52s
ubuntu-20.04 3.1	53s
ubuntu-20.04 3.2	51s
ubuntu-20.04 3.3	51s
ubuntu-20.04 head	51s
ubuntu-20.04 truffleruby-head	1m 29s
macos-latest 2.7	1m 19s
macos-latest 3.0	1m 9s
macos-latest 3.1	1m 26s
macos-latest 3.2	1m 19s
macos-latest 3.3	56s
macos-latest head	1m 34s
macos-latest truffleruby-head	1m 32s
windows-latest 2.7	6m 39s
windows-latest 3.0	4m 28s
windows-latest 3.1	4m 35s
windows-latest 3.2	5m 56s
windows-latest 3.3	4m 2s
windows-latest ucrt	3m 29s
windows-latest mswin	3m 12s
	1h 51m 37s

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