oniguruma: Invalid pointer dereference in left_adjust_char_head()

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 SIGSEGV occurs in leftadjustcharhead during regular expression compilation. Invalid handling of reg-dmax in forwardsearchrange could result in an invalid pointer...