Add one more guard to avoid mixing nil into latest_specs

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@58616 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

lib/rubygems/specification.rb

@@ -943,6 +943,7 @@ class Gem::Specification < Gem::BasicSpecification
   # -- wilsonb
 
   def self.all= specs
+    raise "nil spec!" if specs.any?(&:nil?) # TODO: remove once we're happy
     @@stubs_by_name = specs.group_by(&:name)
     @@all = @@stubs = specs
   end