defined? returns nil for toplevel constant lookup

	* variable.c (rb_const_defined_0): toplevel constant lookup has
	  been removed, should return nil too.
	  [ruby-core:85142] [Bug #14407] [Fix GH-1800]

	From: Gonzalo <grzuy0@gmail.com>

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_5@62568 b2dd03c8-39d4-4d8f-98ff-823fe69b080e