rational.c: segfault on Rational exponent * rational.c (read_num): fix segfault on Rational() with positive but less than the length of fractional part exponent. should be negated to convert to divisor which is a reciprocal. [ruby-core:85783] [Bug #14547]
merge revision(s) 62555: [Backport #14547]
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_5@62562 b2dd03c8-39d4-4d8f-98ff-823fe69b080e