read: don't clear buffer when nothing can be read #1046

Job	Run time
openssl-1.0.2u	2m 34s
openssl-1.1.0l	1m 29s
openssl-1.1.1w	1m 42s
openssl-3.0.13	2m 51s
openssl-3.1.5	3m 25s
openssl-3.2.1	3m 14s
openssl-3.3.0	3m 35s
libressl-3.1.5	2m 8s
libressl-3.2.7	2m 23s
libressl-3.3.6	2m 15s
libressl-3.4.3	2m 15s
libressl-3.5.3	2m 29s
libressl-3.6.3	2m 16s
libressl-3.7.3	2m 25s
libressl-3.8.4	2m 23s
libressl-3.9.1	2m 9s
openssl-3.0.13 fips	2m 57s
openssl-3.1.5 fips	2m 53s
openssl-3.2.1 fips	3m 12s
openssl-3.3.0 fips	3m 14s
openssl-head	3m 26s
openssl-head fips	3m 19s
openssl-head no-legacy	3m 24s
ruby-versions / Generate Ruby versions	7s
ubuntu-22.04 2.7	57s
ubuntu-22.04 3.0	57s
ubuntu-22.04 3.1	1m 3s
ubuntu-22.04 3.2	1m 1s
ubuntu-22.04 3.3	56s
ubuntu-22.04 head	55s
ubuntu-22.04 truffleruby	1m 21s
ubuntu-22.04 truffleruby-head	1m 37s
ubuntu-20.04 2.7	53s
ubuntu-20.04 3.0	55s
ubuntu-20.04 3.1	52s
ubuntu-20.04 3.2	57s
ubuntu-20.04 3.3	48s
ubuntu-20.04 head	51s
ubuntu-20.04 truffleruby-head	1m 24s
macos-latest 2.7	48s
macos-latest 3.0	39s
macos-latest 3.1	47s
macos-latest 3.2	39s
macos-latest 3.3	44s
macos-latest head	36s
macos-latest truffleruby-head	54s
windows-latest 2.7	4m 47s
windows-latest 3.0	5m 4s
windows-latest 3.1	5m 45s
windows-latest 3.2	5m 8s
windows-latest 3.3	5m 14s
windows-latest ucrt	4m 11s
windows-latest mswin	3m 10s
	1h 55m 58s

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