* bignum.c (big2str_karatsuba): Reduce power_level more than one at
  recursion, if possible.
  (rb_big2str1): Follow the above change.



git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@42325 b2dd03c8-39d4-4d8f-98ff-823fe69b080e