Ruby

avoid division by zero

• cvt(): use signbit() instead of 1/d < 0
• w_float(): ditto
• ruby_float_step_size(): unit==0 check shall be prior to divisions
• arith_seq_float_step_size(): ditto
• rb_big_divide(): same as r65642
• fix_divide(): ditto
• rb_big_fdiv_double(): ditto
• fix_fdiv_double(): ditto

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@65751 b2dd03c8-39d4-4d8f-98ff-823fe69b080e