* variable.c (rb_f_trace_var): should not be allowed at safe level 4.  
	  a patch from Keita Yamaguchi <keita.yamaguchi at gmail.com>.


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_1_8_6@17690 b2dd03c8-39d4-4d8f-98ff-823fe69b080e