bigdecimal.c: expand buffer

* ext/bigdecimal/bigdecimal.c (BigMath_s_log): expand buffer size for
  64bit platforms.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@44558 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
* 2014-01-11

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@44559 b2dd03c8-39d4-4d8f-98ff-823fe69b080e