Add test and fix for buffer over-read in pcre2grep#594

NWilson merged 2 commits intoPCRE2Project:masterPCRE2Project/pcre2:masterfrom

NWilson:user/niwilson/pcre2grep-valgrindNWilson/pcre2:user/niwilson/pcre2grep-valgrindCopy head branch name to clipboard