ssl: fix potential exception in servername_cb #1249

Job	Run time
openssl-1.0.2u	56s
openssl-1.1.0l	51s
openssl-1.1.1w	59s
ruby-versions / Generate Ruby versions	1s
openssl-3.0.15	1m 3s
openssl-3.1.7	56s
openssl-3.2.3	55s
openssl-3.3.2	54s
openssl-3.4.0	1m 1s
openssl-master	3m 50s
libressl-3.1.5	53s
libressl-3.2.7	1m 3s
libressl-3.3.6	59s
libressl-3.4.3	1m 1s
libressl-3.5.3	58s
libressl-3.6.3	1m 4s
libressl-3.7.3	1m 0s
libressl-3.8.4	58s
libressl-3.9.2	56s
libressl-4.0.0	1m 5s
openssl-3.0.15 with fips provider	50s
openssl-3.1.7 with fips provider	54s
openssl-3.2.3 with fips provider	55s
openssl-3.3.2 with fips provider	54s
openssl-3.4.0 with fips provider	47s
openssl-master with fips provider	3m 31s
openssl-3.4.0 without legacy provider	56s
ubuntu-22.04 2.7	1m 2s
ubuntu-22.04 3.0	52s
ubuntu-22.04 3.1	1m 4s
ubuntu-22.04 3.2	1m 3s
ubuntu-22.04 3.3	58s
ubuntu-22.04 head	1m 0s
ubuntu-22.04 truffleruby	1m 40s
ubuntu-22.04 truffleruby-head	1m 47s
ubuntu-20.04 2.7	1m 1s
ubuntu-20.04 3.0	58s
ubuntu-20.04 3.1	57s
ubuntu-20.04 3.2	55s
ubuntu-20.04 3.3	58s
ubuntu-20.04 head	55s
ubuntu-20.04 truffleruby	1m 31s
ubuntu-20.04 truffleruby-head	1m 49s
macos-latest 2.7	50s
macos-latest 3.0	36s
macos-latest 3.1	53s
macos-latest 3.2	43s
macos-latest 3.3	38s
macos-latest head	39s
macos-latest truffleruby	1m 5s
macos-latest truffleruby-head	1m 2s
windows-latest 2.7	4m 28s
windows-latest 3.0	4m 55s
windows-latest 3.1	5m 0s
windows-latest 3.2	3m 47s
windows-latest 3.3	4m 7s
windows-latest head	4m 41s
windows-latest ucrt	3m 59s
windows-latest mswin	3m 3s
	1h 29m 6s

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