fix a potential NULL dereference spotted by gcc 8.1.0

fix a potential NULL dereference spotted by gcc 8.1.0

 *) mod_ssl: fix a potential NULL dereference spotted by gcc 8.1.0
    mod_http2: silence gcc strncpy warnings which break compilation in
               maintainer mode with gcc 8.1.0
    trunk patch: http://svn.apache.org/r1831231
                 http://svn.apache.org/r1831591
                 http://svn.apache.org/r1832934
                 http://svn.apache.org/r1832937
    2.4.x patch: svn merge -c 1831231,1831591,1832934,1832937 ^/httpd/httpd/trunk .
    +1: ylavic, icing, jailletc36 (by inspection)

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1833564 13f79535-47bb-0310-9956-ffa450edef68