brew audit: avoid error on head-only formulae

Library/Homebrew/dev-cmd/audit.rb

@@ -584,6 +584,7 @@ module Homebrew
       ]
 
       throttled.each_slice(2).to_a.map do |a, b|
+        next if formula.stable.nil?
         version = formula.stable.version.to_s.split(".").last.to_i
         if @strict && a.include?(formula.name) && version.modulo(b.to_i).nonzero?
           problem "should only be updated every #{b} releases on multiples of #{b}"