Fix for when version is nil

When version is nil, (version <==> other.version) also returns nil
and then the nill object doesn't have a nonzero? method.

I'm not sure what package put me into this state, but, this fix repaired
my environment.

Also, there is probably a more ruby-esque way to do this, but, I'm not a
ruby expert ;-)

Library/Homebrew/pkg_version.rb

@@ -36,7 +36,10 @@ class PkgVersion
   def <=>(other)
     return unless other.is_a?(PkgVersion)
 
-    (version <=> other.version).nonzero? || revision <=> other.revision
+    comp_ver = (version <=> other.version)
+    return if comp_ver.nil?
+
+    comp_ver.nonzero? || revision <=> other.revision
   end
   alias eql? ==
 

Library/Homebrew/test/pkg_version_spec.rb

@@ -54,6 +54,11 @@ describe PkgVersion do
   describe "#<=>" do
     it "returns nil if the comparison fails" do
       expect(described_class.new(Version.create("1.0"), 0) <=> Object.new).to be nil
+      expect(Object.new <=> described_class.new(Version.create("1.0"), 0)).to be nil
+      expect(Object.new <=> described_class.new(Version.create("1.0"), 0)).to be nil
+      expect(described_class.new(Version.create("1.0"), 0) <=> nil).to be nil
+      # This one used to fail due to dereferencing a null `self`
+      expect(described_class.new(nil, 0) <=> described_class.new(Version.create("1.0"), 0)).to be nil
     end
   end