Fix an assertion failure in tls_common.c #830

Job	Run time
run-checker (enable-asan enable-ubsan no-shared no-asm -DOPENSSL_SMALL_FOOTPRINT)	9m 51s
run-checker (no-ct)	5m 14s
run-checker (no-dso)	4m 55s
run-checker (no-dynamic-engine)	4m 55s
run-checker (no-ec2m)	4m 43s
run-checker (no-engine no-shared)	4m 3s
run-checker (no-err)	4m 58s
run-checker (no-filenames)	5m 3s
run-checker (enable-ubsan no-asm -DOPENSSL_SMALL_FOOTPRINT -fno-sanitize=alignment)	7m 40s
run-checker (no-module)	5m 3s
run-checker (no-ocsp)	5m 0s
run-checker (no-pinshared)	4m 56s
run-checker (no-srp)	5m 2s
run-checker (no-srtp)	5m 7s
run-checker (no-ts)	5m 0s
run-checker (enable-weak-ssl-ciphers)	5m 4s
run-checker (enable-zlib)	4m 57s
	1h 31m 31s

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