By definition, the logarithm of 0 is negative infinity.  This is a
pole error (cf: cf: ISO/IEC 9899:1999 section 7.12.1 paragraph 3) and
of course, cannot fit into an `int` value.  We have to resort to
INT_MIN.


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@65743 b2dd03c8-39d4-4d8f-98ff-823fe69b080e