oniguruma: Out-of-bounds stack read in match_at() during regular expression searching

An issue was discovered in Oniguruma 6.2.0, as used in Oniguruma-mod in Ruby through 2.4.1 and mbstring in PHP through 7.1.5. A stack out-of-bounds read occurs in matchat during regular expression searching. A logical error involving order of validation and access in matchat could result in an...