Raise error if spec is nil

With parallel test-all, the spec is sometimes nil.
To debug it raise more detailed error.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@57956 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

lib/rubygems/specification.rb

@@ -1128,6 +1128,9 @@ class Gem::Specification < Gem::BasicSpecification
     native = {}
 
     specs.reverse_each do |spec|
+      unless spec
+        raise Gem::Exception, "unexpectedly spec is nil: #{specs}"
+      end
       next if spec.version.prerelease? unless prerelease
 
       native[spec.name] = spec.version if spec.platform == Gem::Platform::RUBY