Suricata lua/tls: null dereference in TlsGetCertInfo

Suricata is a network Intrusion Detection System, Intrusion Prevention System and Network Security Monitoring engine. Prior to version 7.0.16, the Lua TLS certificate information helper could dereference NULL certificate fields when a Lua script requested certificate information for TLS traffic where some certificate fields were absent. Crafted TLS traffic processed by a deployment using affected Lua TLS scripting could crash Suricata, resulting in denial of service. Version 7.0.16 contains a fix. As a workaround, avoid Lua scripts that call TLS certificate information helpers on untrusted traffic (`TlsGetCertInfo` function), or update scripts to handle missing certificate fields where possible.