math.c: adjust prototype of math_log

* math.c (math_log): adjust prototype as method function.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@45794 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
math.c: check domain of base argument

* math.c (math_log): check domain of base argument too.  raises
  Math::DomainError instead of returning NaN if the base is less
  than 0, and returns NaN instead of -infinity if both of two
  arguments are 0.  [ruby-core:62309] [Bug #9797]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@45795 b2dd03c8-39d4-4d8f-98ff-823fe69b080e