* lib/cmath.rb (log10): raised exception when the given number is a negative real.
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_1_9_1@23780 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
merges r23658 from trunk into ruby_1_9_1.¶
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_1_9_1@23780 b2dd03c8-39d4-4d8f-98ff-823fe69b080e